I've said this about a thousand times, and i'll say it again.
If you are having trouble finding a main, make a list of wich characters you like best.
Here is a list of good rush-down characters:
-Cyber sub-zero- notably, if your new to him you'll shit out bombs from rushing down with 33 or 34. But it's able to overcome.
-Johnny Cage- Good Frame traps, and good rushdown.
-Kung Lao- Good rushdown with 24low hat or 24OH or 24 throw,don't know too much about him but he builds INSANE meter.
-Sonya- as FrothyOmen said, she's underused and great at rushdown with that f*cking military stance. Watch some Morty Seinfeld vid's.
-Liu Kang- He can rush down without a doubt man. low starter string, and an overhead string. Atl_Redd is a good liu kang player.
-Kabal- Oh god...Kabal. Good rushdown, and reset with knee to prevent wake-ups and continue pressure, build's WAY too much meter with his nomad dash cancel(sorry the name escapes my mind right now)
-Sub-Zero- Now..He CAN rushdown, and it's his best option. The fact that he can tell you"you want to lose 30% of your life, or would you like to not be able to move?" is great. If your opponent blocks one of your string's you cancel into a clone to keep them out, then you go in for more pressure. His Corner game is the best, hard to get out of against a good sub-zero.
-Smoke-This isn't your typical rush-down character. He only has OH strings but the fact that you can do B23, d3, d4, B23 is incredible. He has a fast sweep and great pokes, using all of this to your advantage can mind-f*ck the opponent.
Those are the rush-downs that I know.
Pick a character that fits you. Gameplay, story-line, outfit/color, everthing.
Best of luck to you man!
~Hound.