Ugh I have so many favorites. It's hard to decide. Honestly if I had just stuck with one and put all my dust into it, I'd probably have all their legendaries and epic cards unlocked by now, but I can't stick with any long enough to do that.
I love Lyonar for their epic heal combos, though admittedly, that requires some of the harder to obtain epic cards to pull off to full effect. Still, it's beautiful when it works. High HP minions with healing to keep them alive, while doing damage to enemies simultaneously. Then of course Divine Bond to deal the killing blow of sometimes 15 or more damage easily.
I like Magmar just because of the
value of their units. It's not enough that there's a 4/5 Veteran Silithar on the board, but you've gotta kill it TWICE or it'll respawn. Even the 2/3 Young Silithar is annoying in that way. Spirit Harvester is just the Abyssian ender, dealing 1 damage to minions each turn and having great stats to boot. Makantor Warbeast, a 4/4 with Frenzy AND Rush is simply game-ending strong, nothing much else to say about it. Silithar Elder of course being an 8/8 which can respawn, but also summons a copy of itself each turn, which summons a copy of itself, which summons a copy of itself...if your opponent doesn't deal with this quickly, it's soon becomes a doomsday scenario. I've summoned one of these on turn 2 before. Needless to say, it was an instant concede.
Of course their new Legendary, the sister known as Earth Sister Taygete is easily the most broken of the bunch. Not much else to say except ouch. Card will probably be nerfed, but for now, it's godmode.
Abyssian is one of my favorites as well. It would be more of a favorite of mine except that everybody seems to like to play it. I'm not super complaining about this though, because if you see it a lot in ladder you can just play Magmar and get free wins. But even though it's overplayed, there are so many cool things about it. The Deathwatch mechanic, the Shadow Creep, a lot of their units are just really neat. Their Legendaries are definitely some of the coolest in the game. The combo potential between so many of their cards...ugh it's all just so disgusting. I'm planning on spending my next ~5000 dust craft an Abyssian Dying Wish Combo deck with Unseven, Vorpal Reaper, Reaper of the Nine Moons, and Dioltas. When it's done I think it's going to be shoot yourself in the face scary.
Right now I have an Lilithe Abyssian deck that kind of hinges around Grailmaster and Dark Sacrifice. The way
Grailmaster works is that she's a 7 cost minion who gets more powerful each time you summon a new minion. So playing that card, then doing the mass spawn thing that Abyssian is so good at is pretty much an instant win condition unless they can deal with Grailmaster in a single turn, which is hard to do because I place it on the edge of the map.
I have another Abyssian deck, but this time with Cassyva, which frankly I think is a bit genius. You see, Cassyva is the hero who centers around the use of Shadow Creep, however Shadow Creep by its very nature is a lategame mechanic. You have to build it up over the course of a match for it to be truly effective, and there are only a few minions/cards that can apply it. So you're very limited until the later stages of the battle, after which you start turning the whole board into toxic poop. So to counter-act this early game weakness, I made it a hybrid deck with Shadow Creep and Mechaz0r. So summoning Mechaz0r is the win condition for the early to midgame, which your opponent will be scrambling to stop. However, even if he does somehow stop that, by the lategame you will have usually bought enough time to win with Shadow Creep. It's a pretty nasty combo.