For the convenience of other forum users, recategorizing your hero roster:
5* - Magni & costume
4* - Boril
3* - Graymane
5* - Cobalt, Skadi, Glenda, Richard, Snow White
4* - Jott, Grimm & costume, Kiril
3* - Frosty, Vodnik
5* - Telluria
4* - Kashhrek
3* - none
5* - Zeline, Lianna, Zocc, Elkanen
4* - Peters, Almur, Hansel, Gadeirus
3* - none
5* - Gravemaker
4* - Sumitumo, Wilbur
3* - none
5* - Azlar, Noor, Khagan, Grazul, Marjana (maxed?) & costume, Reuben, Anzogh, Santa Claus
4* - Boldtusk, Sir Lancelot, Kelile (maxed?) & costume
3* - Rudolph
5* - Hel, 1st Clarissa
4* - Rigard, Tiburtus (almost)
5* - Grimble, Thoth-Amun, Sartana, 2nd Clarissa, Obakan
4* - Merlin, costumed Tiburtus
3* - Jack
5* - Norns
4* - Gullinbursti, Wu Kong, Jackal
3* - Kailani, Bane, Gan Ju
5* - none
4* - none
3* - costumed Kailani, costumed Bane
I may have missed some heroes. Too difficult to recategorize using my mobile phone. It doesn’t matter much because of my suggestion below.
As to your query which 5* hero is to be maxed, my answer is NONE… yet. Why, you may ask? I find that your army of maxed 3* and 4* heroes are wanting. I think it is wise and prudent to work on your 3* and 4* heroes first. Level and max at least 3 rainbow teams of 3* heroes and at least 3 rainbow teams of 4* heroes. This means that you have to have 3 maxed heroes for each color for 3* as well as to your 4* heroes. They will help you improve gaming experience in the other facets of the game. Remember, the resources of maxing one 5* hero is enough to max two 4* heroes. Hoard those 4* ascension materials until you have established your army of 3* and 4* heroes.
On your 3* heroes, work on the costume versions of your Kailani, Bane, Jack, Frosty and Vodnik. You will find them useful.
On your 4* heroes, you may also work on the original and costume versions. They are relatively easier and cheap to max and emblem than 5* heroes. Work on Boldtusk, Tiburtus and costume, Grimm and costume, Peters, Almur, Hansel, Kiril, among others. Prioritize healers and fast heroes.
Read a lot of guides offered and posted in the community forum. The search tool here will be your best friend in the coming hours, days, weeks and may be months.
And lastly, always always always find fun playing and/or reading here, even if other forum users may also provide their insights different from mine.
Just my 1 cent.