Elves are easily the most fun. There's so much to do with them. Layer their attack all the way up as a tree whisperer holds em for big stacks and elves are great.
Then again, orcs are fun how you can get a 5k stack in a day no problem and run around with wolves and shamans to make up for the immobility.
As my card collection grows, goblins are becoming one of my favorites. They lack in outright offensive capabilities but more than make up for it in diversity and money making.
However, trolls would probably be my favorite altogether. They're easily the most balanced with an edge towards combat. The rune dancer is one of the best units in the game in my opinion. At 4 mana and cheap deployment and can decimate stacks, especially of goblins. Then you have the Crone. Pair her with an assassin and you can take down any lord with a little luck. 150 for their strong base unit is better than any other race. Tangle mage is cool when you want to help your friends and the bridge witch is a super dragonhelm knight. And finally the serpent charmer. What a great money maker. Trolls for the win.
I can't say I really have a favorite hero thoigh, as they all play such great roles for different situations, but I do love the cowardly noble after a couple big wins. It's always nice to get a gold bonus after some big victories and he's a great starting hero as he gives you 50gold (after rebate) when you play him right off the bat for 4 mana. That's a steal, that's a deal. And the goblin banker is a must have for any 72+ hour games.