People like the classes that offer a 50% mana start, those are all very good by default.
A few hero talents that make their hero stand out are Lightning Strike, ones that confer predictable status benefits like Barrier on color matches, and ones that summon replacement troops for dead allies.
Lightning Strike churns the board, and barrier-on-brown keeps your hero alive through ridiculous situations. Titan is considered objectively best by many because it offers both of those AND a fast mana start.
But some other classes like Bard, Archer, and Priest offer just enough utility beyond their talents people like to use them too.
I don’t agree with awryan on Thief yet, but I do know as a defensive class it’s very strong. Similarly, Dragonguard can be a pain to deal with.
So I guess I need to sit down and crunch which classes have the talents I like, but it’s so easy to just play Titan all the time I haven’t. Maybe if it didn’t cost gems to experiment I’d be more willing to branch out, but meh.