Beasts, mechs and demons standout so far. Dragons and elementals can be killer but seem to be too dependent on a few key cards. Ysera really helps solve that issue for dragons. I tend not to play murlocs so I don't have much of an opinion on them.
There are some other heros that seem to have more promise now.
Tess if beasts are in play. I've only played Zeph once but he helped get some key goldens. I do like Silas for sure. Reno and Galakrond I can't seem to say no to for the fun factor. Galakrond doesn't even have to go big, you can use him in small upgrades to get the key cards you need all game. I haven't yet tried Vol'jin but look forward to after the above tip.
I'm really liking Malygos to assist in finding the key cards you need throughout the game.
Appreciate the Parrot/Buster combo tip earlier in this thread. The Parrot and Barron were both golden and reborn on the Parrot.
My only real beef with BG is the MMR pools. There is too much of a gap between pools. I consistently go back and forth between runs where I can't lose and then runs were I can't win. It's not just a streak, it is skill based. I am outclassed in one pool and get my ass kicked hard. I need to just keep getting killed until they drop me down... at which point I am above average skill and kick ass. 6-10 games in each stint. I really hate it.
It's hard to improve on my skills with the system. The tactics that I have learned that do well in the lower pool consistently are not good enough in the higher pool... not even close.