Sivir, Corki, Ashe, Cho'Gath.
Sivir pushes like no other, by the third creep wave, they are constantly at their tower. They try to come up to last hit, your bouncing blade harasses them while you focus their melee creeps. Grab a single level of spell shield, so that if they get you within a single nuke of death, you can save yourself (great against ashe, wait for the level 6 arrow, spellshield, gg). And her boomerang nuke is one of the strongest in the game, both for pushing and harassing, if you are good with it.
Corki is amazing, doesn't matter what items you get. Flash bomb to harass and farm backline. Valkyrie to from front creep line back to burn em down. Starting at 6, he's got a long range harassing/farm nuke for 20 mana. Amazing if you're good with aiming it.
Ashe, the mid solo in 95% of low ELO games. Boring, but amazing nonetheless. Frost arrow to keep enemy champions at bay when they're trying to harass, or to chase em down better, volley for farming and long range harass, and a passive that most people consider **** but I think is great to complement the large amount of last hitting you will be doing. Finally, getting to 6 early to arrow your own lane for an easy kill, or launch into another lane to help them out is great fun and can earn teammates a kill or two.
Cho'Gath is just an amazing solo. I still remember a game that I soloed against a Ryze, we both got to 6, he ulted me, I just kept on farming his creeps.
I haven't used Annie much (when is week 2 of champ rotation getting here so I can try her again?) but I've been in plenty of games where she rocked both her own lane opponent, then comes down to dual lanes and pulls off double kills with an energized ult. She can be scary as hell.