Depends on play style. My thought is that purple 3* has the weakest selection as a group, in that their specials are all offensive against the foes, with none having effects directly benefiting benefiting their own team. But I like a variety of choices. YMMV. Tyrum is a very good striker in particular for example. Balthazar hits hard but is squishy. Don’t even bother with Renfield.
Best is probably green. Belith heals and dispels, while Brienne’s special likely makes her the best 3* there is. Isshtak isn’t too hot once you get better options, but Carver and Berden can be pains even for a team of 4*. Tuck is a limited healer and mana booster. Not my first choice for primary green, but for a hard fight can be a decent choice.
Blues, look to Gunnar and Valen. Spirit Link from Gunnar is a lifesaver, and Valen’s a good striker at that level. Ulmer’s decent. Graymane looks cool but isn’t great, and Karil doesn’t even look cool.
Yellow, Bane of course. Even worth working up a second one, especially for the Pirates event beginner levels. Kailani is another Spirit Linker, making her worth grabbing. The others aren’t all that noteworthy. Not bad, but mostly useful as fodder for better heroes if you have them.
Reds. Hawkmoon is one of the three 3* healers. Nashgar and Azar are both fast strikers and nice in a fight. Jahangir can do a lot of damage to an opposing force, but is so slow he can be hard to keep alive to fire.
Hope this is useful.
Edit - typo fixes.