The standard advice about setting the game to easiest and one round is correct. The trick with just walking forward and/or hitting hard kick works for pretty much everyone... until you get to Seth. It seems to work particularly well for Crimson Viper since her forward and roundhouse is an armor-breaking two hitter.
For some reason, the fight against Seth using Ryu was way harder than I expected, and way harder than with any of the other characters. Very strange. The most annoying thing about Seth is how he seems to ignore his vulnerable period when he's coming back down out of the air (either from a jump or DP).
Least effort to beat Seth? Bison, no question. At the beginning of the round, jump straight up and then immediately slide-sweep. When he starts to get up, jump backwards and slide-sweep again. Repeat until he's toast. This works obscenely well on just about everyone, in fact - at least on easiest. Also, I used Bison as the final character to beat Akuma and thus unlock him. I recommend this approach.
Crimson Viper and Abel both turned out to be more fun than I expected. I'll have to look into them more. I had mistaken Viper for a charge character, but she's not.
Interesting question: Who/what is Seth? When Abel beats Seth, his winning speech says something that I also noticed: Seth has Abel's face. Sakura's ending hints that there's more than one Seth. Bison's ending explicitly shows multiple copies. Crimson Viper's ending implies that there are 27 copies of Seth (the one she just beat, plus 26 more.) It's all rather sci-fi, for a Street Fighter game.
Funniest thing in the game so far - When Sakura beats Sagat, her winning speech is: "I totally understand your obsession with Ryu. Isn't he a dreamboat?" ;D