MVPs: Noril and Dölgöön, both are top picks for more than 1 tourney type.
Even Shrubbear can be used for more than 1 tourney (but is mainly a rush hero I think)
Mnesseus wins a lot if you get his costume, as you can then choose between dispel and cleanse as a utility hero. Can be used in any tourney, but not necessarily a must have, the standard version dispel is of course most valuable for buff booster tourneys, his fast speed makes him also good for bloody battle as a sniper.
As to healers: I think with the 2 you have you´re good for rush (Faiez) and buff booster (C-belith), Grevle is best at bloody battle in my opinion (faster than Faiez and the attack boost is more useful than belith dispel/anti-dispel).
Gramps: rush specialist, I would say it´s a matter of playstyle, I personally don´t use him, but wouldn´t say he´s bad either.
By-Ulf: buff booster specialist, again may work for some people, but I don´t like him and feed him away every time I get him.
Hisan, just a sniper, Mnesseus is better (because of his utility), I see no use for him as a pure sniper. I feed him away.
Muggy no use I can see…
As to the ones you have and I haven´t yet mentioned: Wouldn´t feed any away.
Isshtak is OK for his def down, especially on offense, may loose value when you get costume for brienne.
Gnomer is a defense specialist (not offense, speed less relevant, damage too low) for wing position (especially bloody battle, maybe also buff booster, if he gets buffs from someone else)
Brienne, wait for the costume, great defdown hero (biggest green def down in the game I believe), especially for titans
Regular version can be used for buff booster as well.