The shadaloo kings and Seth. My main and alt are Bison and seth respectively and I dabble in Sagat, Vega and Balrog.
Up next is Gen, Viper, Abel, Gouken, Dhalsim, and Akuma, in that order. Gen and Viper are execution heavy characters that require a lot of training to win with. Much respect to all those who main them.
Abel is one of my favorite new cast members. A really solid, well balanced character with enough weaknesses to balance his ridiculous rushdown.
Gouken in the right hands is amazing, fantastic juggle setups and requires solid spacing and good reading of the opponent and this yields huge damage and high stun.
Dhalsim is another character that requires excellent spacing, mindgames, and proper utilization of all of this toolset to win.
Akuma is awesome just because he's the only shoto character in 4 that I really really respect. The guy's mixups are insane and he has a great vortex but all of this is balanced with a glass jaw. he doesn't have anything cheesy like FADC ultra and relies on mixing the opponent up and keeping the pressure on. also execution heavy. mad respect.
In 4 I pretty much like any character that has significant weaknesses to make up for a solid toolset, is execution/spacing/mindgame heavy. Ryu has to work hard to win but he's TOO well rounded imo. He lacks enough weaknesses to really get the respect that someone like Gen or Viper deserves.
I'm speaking strictly of SF4, but in ST or CE, Bison and no one else. TOD FTW.