I've absolutely fallen in love with playing Legion Commander lately. But I was terrible as her when I started. So even though I'm actually a pretty competent player now, my winrate on her is... pretty abysmal. (Just over 30%, last I checked) I'm working on bringing that up, but there's a lot of ground to cover.
Other than that, Luna and Slark are my favorite carries. Crystal Maiden and Earthshaker are my favorite supports, with Earthshaker probably being my favorite hero in the whole game.
I generally try to play all roles in cycles, but I've found that I'm usually the most at home in the offlane. Legion Commander, Dark Seer, and Earthskaker, as examples, are just so much fun to play solo offlane and use that sweet, delicious solo exp to start the gank train. But... don't ask me to go position 5 support or mid. I can generally hold my own there, but I'm nothing special in mid, and I'm terrible at consistent support play. Warding, stacking, pulling, dewarding, smoke ganking, rotations... it all slips my mind far too often.