There are some heroes in my roster that i cant use yet that would be on the top 1 without doubt (Hannah, Costume Leonidas, Alexandrine) However, as im still working on my roster i will name just those that add value right now. One of my most used heroes is Grevle, i use him in a green stack with Marcel. He helped me to go throw hard events and to advance pretty much further on Atlantis and Valhalla.
Another hero who changed my level was Bertulf. Thanks to him i improved my titan damage and overtall i can beat some hard stages and bosses.
For my humble roster, Proteus - I can use him in both 4* and 5* tiers of events and tournaments, still use both of them every war. Useful against anything that can gain mana - even if Clerics and Monks can resist.
The ability to shut down 3 enemies at a time is just amazing.
Krampus is my most powerful hero, probably the most desirable of those on my roster for most people, transformed my defense, and is the first five-star I limit broke.
That said, the most versatile and probably all-around most useful hero for me is my very first five-star ever… Onatel. Beefy stats, mana control, self-heal elemental link, and awesome artwork to boot. She works at any spot on defense, and she controls mana well enough to give me a chance in almost any PvE content. On offense, I have lost track of how many raids have come down to Onatel against two or three opponents, just doing her thing until I get the win. I even used Google credit to buy the last PoV pass, in part because I could get the Onatel avatar. I use it now, despite being a dude. Can’t believe how much I lucked out to land her when she was HotM on like four pulls.