Voted for Gengar, although I'm more into Haunter (because of the anime and owning only Haunter in the games due to lack of people to trade with). Gengar's all right, but I wouldn't put him on my favorites list, personally. (Although he is my second favorite Ghost-type. I just can't say no to its standards, even though I really should. ♥)
And don't get me started about Gastly. Stupid puff ball won't leave me alone in the Pokemon Tower (I'm too cheap for Repels/the kind of person who spends thousands of units on Poke Balls and Potions instead), and I usually can't do much with Gastly at all, what with its best moves for in-game training being Night Shade and Confuse Ray until I can finally get it to evolve. >_> (Yeah, I know it learns Shadow Ball, but only at level 29. In DP! ;_;)