Soul Exchange 12
My view on this is from that of a Top 200 alliance player. At my level more heroes are playable than in the Top 20, but I am mostly interested in heroes that make me better, not that keep me floating where I already am. And I suggest the same view for everybody, no matter where they are at. The big guys obviously don´t care much about anything here, they´ll probably select something for the museum or a triplet for a hero that they like both copies they already have, most likely one where they have the costume(s). Players in lower parts of the game will have more heroes of interest, but since they pay the same price, I would still suggest to look at the best hero possible, rather than something that is affordable and barely good enough for a top 1000 alliance war.
Also never expect a meta-hero or anything that even gets close to that. We are talking about heroes you can get without paying real money for!
So generally speaking, what is a good hero to pick from SE (or same principal at a lower level also in FS)?
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A hero that is simply super strong (very rare in SE and never in FS)
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A hero that fills a specialized niche, like a titan specialist, but is otherwise useless. (Usually the option for people with a higher quality roster)
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If you already have a costume for one of them a duplicate may be a great idea (as you get a much more powerful hero for the same price) or if you have a base version and can pick a costume here (you basically get 2 heroes with 2 cstumes for the price of 1), that may make a lot of sense.
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Most importantly: a hero that makes you better at something that you want to be better at. And that can be nearly anything depending on interests.
And, again, generally speaking, what is a bad hero to pick?
A pure damage based hero, because those get outdated the fastest and since all heroes in SE and more so FS are already old, you start out with something that is outdated by the time you get it and gets worse from there.
If you put the above two things together: Pick a hero with a timeless skill, that makes you better at something important. If nothing like that is available, maybe go for a healer, they at least age slower than hitters, especially if they are %-based.
The best heroes are of course (as they should be) in the highest category, so the general recommendation is to save up more heroes and go shopping for a 20 next time, if you don´t have them this time, but it is definitely worth looking into the lower categories for good deals or those aforementioned niche heroes. Also SG has made errors of judgment before and placed amazing heroes in lower categories and garbage in the highest one. So look at all of them and sleep over your thoughts for a night or two. The price doesn´t go up just because you wait a while 
10 souls: Nothing exciting this time
Mortimer (TP always at 4-80: 873): Horrible stats, but interesting skill. Silence 3 at avg. speed is nice! Similar to Cupido, if you know who that is.
Queen Guowang (TP: 862): Horrible stats. Will probably get a costume soon, but you have to pull her again then and do you really need 2 of her? Hitter without stats is just not a thing.
Neema (TP: 891): A hotm that was never great and is way outdated on top of that now.
Kettle (TP: 940): Worst hero released in years! Kinda funny we had the second worst Goblin last time (my choice for worst pick in SE12, if you remember) and went downhill from there… thanks SG!
CHeimdall (TP: 959): People love revivers, so this one will be popular. I don´t agree, but that won´t stop anyone, so have at it 
Similar quality reviver as CMother North, which I don´t think is a good hero either. But he has the best stats in the 10 soul category. The original is not of interest.
15 souls: 2 absolute top picks here, some others OK
Xiong Shannjun (TP: 874): The Cocaine bear used to be a horrible hero, then got buffed up to not-very-interesting. As a bonus, he´s got bad stats. Thanks SG!
Winnie (TP: 1027): More than 1000 TP (second best stats after Ma´at) I got her when trying for Xiamara (see later) and hated her, cause nobody needs more slow healers lol (maybe Grimsteel) anyway then later they buffed her with another passive to gain mana per buff and suddenly you play her together with a yellow avg speed (or faster) hero that gives 2 buffs to Winnie and suddenly she´ll fire in 9 tiles (with a 20% mana troop). That makes her perfect for 3-2 teams. Only problem is: I cannot play 3-2 anymore cause the damage just isn´t enough on the 3 stack 
So I hardly play her at all anymore outside rush, which is a shame as I really liked my Kara-Winnie power ladies as bodyguards… But if they guys you protect can´t get the job done, what´s the point? And by herself Winnie is just another slow fattie… But if you either have strong enough hitters to be able to kill stuff with 3 or play low down enough where that is the general rule, then she´s a great asset, given you have a partner for her.
Serandite (TP: 867): Used to be one of those third gen OP monster Ninjas, but by now he´s rather harmless, with the bad stats his damage is not great, nor is the DoT also got healing reduction, I wouldn´t call him useless, but also not exactly exciting. You know damage and old stats is
Waterpipe (TP: 887): Used to be a scourge, meta tank for a while who led to the invention of Lunar heroes to avoid a nerf, which in turn messed up the entire minion game. Kinda lame as a tank nowadays, but still a very interesting offense hero (because nobody in their right mind puts a Lunar hero on their def), especially interesting of course in rush, but at avg speed also playable at normal speed. Can also be a way to control nasty PvE bosses like in the treasure quest for example.
CMa´at (TP: 1109): WOW! A S5 costume! More than 1100TP (first time a hero above 1k), now those are heroes we like to see in SE! (see also Winnie). Amazing deal at 15 souls with a fast speed DD to all and a bunch of passives and whatnots… Now all you need are some good, fast green hitters in your roster and you have an easy choice. Costume and Original are rather similar, so no big deal which of the 2 you prefer. Just to mention, when they decided on the heroes in this SE, S5 costumes didn´t have the buff yet and her stats would have been below Winnies´ stats. So this one was a “mistake” by SG and I am pretty sure they would not have offered her with the new CB in mind.
20 Souls: All of them are good, some may be better
Deadboot (TP: 940): The def wither made him until recently part of many meta MT teams. Still a good choice of course, but probably not part of the meta team anymore? Besides the titan niche, he could be used for example as a taunt counter. Nice to cover a second niche with one hero.
Kushanku (TP: 875): The stats are really bad for a 20, but the skill is still interesting. Ninja speed is the fastest in the game and mindless is timeless. Great counter for Bearnie/Fergus etc. So probably a popular choice for top players? Remember what I said about niche heroes earlier?
Black Caesar (TP: 987): As you´d expect from a 20 soul compared to a 10 soul pick, he beats Neema by miles in exactly everything. Stats (3rd best in SE13 and a bit better than anything in SE12, which of course should be expected 3 month later in the highest category), damage, effects all of it. Still not sure that makes him a top pick, but at least worth considering.
Odette (TP: 955): Tough one to judge I find. On the one hand a hero that is annoying to fight, on the other a hero I never really cared about not having. Super dodge is fun (can even dodge tiles!) at least for the 4 allies, her own dodge is crappy dodge (only damage dealing skills, which means dispel without damage cannot be dodged), Also the damage is conditional and random, so as I said, it´s complicated. But interesting.
Xiamara (TP: 980): One of the heroes I was super excited about when they came out and never lucked out getting (even spent a little money trying, which not a lot of heroes can say). Back then Granny was a meta tank and Xia totally annihilates Granny, of course Granny isn´t around anymore as tank these days, but she would work against any tank which boosts the allies mana in any way, but especially if it´s a mana generation buff (turning every single tile hitting the target into a damage bomb. Say like Grimsteel for example.
Recommendations:
Best heroes: CMa´at, Waterpipe, Kushanku, Xiamara
Best deal: Since 2 of the above are in the 15 category I will repeat them here: Waterpipe and CMa´at
20 Souls: The 2 top picks are my favs, but all are decent.
15 Souls: The 2 top picks of course are the best deals, Winnie could be an option for some
10 Souls: CHeimdall and Mortimer are OK, but none are amazing choices, I would probably save up souls instead.
Purple: Deadboot
Yellow: Kushanku
Red: Black Caesar
Green: CMa´at, Xiamara
Blue: Waterpipe
Feel free to add or disagree as much as you like
Here is the guide to the current FS if you want to compare the heroes there:
Abbreviations:
SE = Soul Exchange
FS = Fated Summon
CB = Costume Bonus (C… = Costume …)
EDD = Elemental Defense Down
DD = Defense Down
DoT = Damage over time
HoT = Heal over time
AQ = Alliance Quest
MT = Mythic Titan
I hope that covers it, if not let me know
