Oh veeeery good question @JAWS_3D! 
But in order to answer you, I have to take with me a long back in time 
When I’ve started to play in November 2018, I tried a green elemental summon (all of us started from there I think…), 'cause the odds for a shiny 5* were better, and I’d loved to obtain that powerful Lady Elf with a wooden bow…and so Horghall popped up
My first 5* Hero! So excited! With patience and love I levelled up him…yes, he was slow and he didn’t carry the bow…but who cared, he was a 5* tough!
Then I realized how painful was playing with that slow tree…I was sure to hear him humble every time after he fired his skill (so 1 time…): " Buraruuuum…"
Then during Christmas, that gave me Buddy, the Hero I used everywhere for a long time, as I gave him +20 nodes…I’ve made a ×10 pull to summon the green hero that I was admiring at the top of the summons box: Evelyn! She was beautiful, she was fast, she was not a slow humbling tree…
…and so 2 Elkanens popped up! Ok…no Evelyn…but they were green in green shiny armours…and they were FAST!!! I’ve maxed the first, and then after 2 months I’ve maxed the 2nd 
Regrets? Oh sooooo many, my friend…but you know, I was a noob at the time (I’ve found the Forum larer only…)
Who is really to blame for my 2 maxed Elkanens? HORGHALL!!!
Sorry for my poor english, it’s not my native tongue…I’m from Italy!
love the story!! Poor Horghall 
happy leveling!
non è necessario scusarsi 
I say Kingston. While the burn resist is very nice and useful, the lower attack on 3 is often overlooked and very effective. He is on my Titan team for that reason.
Max HP reduction seems to be a theme coming out for new heroes and Heimdall is the only counter right now. I think he deserves a high priority
Speak of the devil, is this you?? Are you sure you don’t want to put in Horghall at tank first?? 
PM me on Line and I can send you the attacks, jaws_3d
Horghall of course!!
Nah… just kidding. Bunch of great greens you have there. And when it’s a close call I would always lean towards more variety. So even though a second Kingston would be useful for sure, Heimdall, Morgan or Ratatoskr
Edit: Didn’t see he was already maxed. Makes my bad joke even worse.
Morgan Le Fay. She holds up much better than Lianna in the long run imo and since she lasts longer she is likely to inflict more damage on your opponents. Morgan is firepower, just different than a Kingston or Lianna but wrecks opponents all the same. Her special can be applied to multiple targets over time. Stat wise she is very well balanced. Her DoT is great and her life drain can make her immortal under the right conditions. Currently mine is doing 854 DoT over 4 turns on offense. On def that number creeps over 1000 DoT over 4 turns. That’s a crazy amount of DoT. She softens up targets for other heroes but she is also quite adept at taking others out on her own. With her DoT life drain Morgan is quite durable. Good tile damage as well for titans and her awesome DoT doesn’t give a crap for other heroes def stats and defensive buffs. Actually she is my favorite hero in game. Love my Morgan Le Fay. With great DoT, life drain, durability, and tile damage Morgan is an easy choice for me.
I go with Lianna other than a 2nd Kingston if for no other reason than Trials.
Plus since you don’t have a Lianna yet why not max her. I’m pretty sure the others will get their chance and she’s the wing you really need to watch while taking out the tank/Flanks. She used to sneak up on me quite often and …Damn killed my healer??