You probably won't believe it, but mine was secret pally. I use to love battling them, with my Dragon priest.
I don't know, l just found class battles more fun. Amazing control Warrior battles, from both side perspective. Midrange hunters, tempo or mech mages, freeze mage battles I was expected to lose, but somehow won. I have a hard time thinking of a bad class to play. Maybe rogue, not because they were bad, just because back then I just didn't play it.
I would honestly have to say the current one. It's a mix of everything. People complain about Jade Druid and Priest, but there's so much going on. Demonlock, aggro Druid, all forms of Warrior.. there's just so many viable options. And it's not an "aggro meta" or anything that can necessarily be pinpointed. A lot is working right now, it feels like you can use any archetype and have it work.
The only problem with secret pally era, was alot of people joined the band wagon. Not that it was op, because alot of decks could and did beat it.
Aggro shaman post first transition to wild. Because it was only true high tier deck that survive it almost full intact. Blizzard failed to spot that, even though pros mentioned before it happened. True cancer deck. Rest so called cancer decks hade Were ok, just a challenge to be taken on.
Probably Wallet Warrior, back when the game was like 10% RNG instead of RNG-fiesta we get now. BUT, post Un'Goro was pretty amazing as well. Every class had at least one high tier viable deck, with some classes having multiples, and most of those decks were 26-27 cards + some tech choices, so you never really knew what you were facing.