I forgot to mention my own favourites!
Before 5.0
AST - I love synergist and saboteur classes. I don't know why, I just find them exciting. So, AST's synergist properties were my jam. I thought the card system was really intuitive and user-friendly. I also really liked the different effects each card had. I actually had fun memorising those and the combinations Royal Road when it first came out. It was just so exciting. The sounds were really nice to me, too (which can weirdly prevent me from playing a class), and I thought the spell-casting animations were nice, light, and not too slow.
BRD - I had only recently picked up (like... 6 months before SHB came out or something like that), but I ended up choosing it as my second main for pretty much the same reasons as AST. I really liked that BRD had potential to not just be "a mindless DPS," because one could somewhat strategise their songs to better aid the team in different situations. (I say somewhat, because of CD. And... because I don't parse or use stats so... I have no idea if it's just a placebo thing for my own ego, LOL.) I really like how mobile it was as well as its animations (like how it flips sometimes. Yesss!).
DRK - The story was what got my hooked initially. After a while, I just thought it felt and sounded and looked so good play. It was really satisfying to be able to take on a mini boss fate by myself and just... slice-n-dice away. XD I didn't get far in levelling, though (at like... 60?). So, I can't really comment too much.
5.0+
I really don't have one at the moment. Dancer is like... a pale version of 4.0 bard. It's my "best" class at the moment and it was fun in the beginning when I didn't know what I was doing. Now it's... it's okay. It's a little boring. To me, anyway. I like the animations a lot, I like the sounds a lot. I do enjoy the uhm.... steppes, seeing the character dance in a battle like that is really satisfying. If some of bards songs were conducted like that, I think it'd be neat. BUT ANYWAY.
I find the response so far to be rather interesting because there is a frequency of some classes being mentioned more than others. At least... thus far. So, I'll be curious to see how it continues transitions as people respond more. /Nods/ I think I might actually give them a shot because they're appearing so frequently. xD