What champion would you play to climb out of bronze (smurf or not)?

To answer this question, one must realize what needs to happen in order to win games at lower elo.

What you need is damage for the most part. Not saying you can't win otherwise, but this is the safest option. At the end of the day, you need to kill the enemies guarding the towers to get them down, which means you need damage, so you pick something that deals a lot of it.

As a sidenote, you can also pick more supportive things, that still do a good job at pushing or forcing enemies out of lane. Janna W max for example is very fun to play.

Now, with all these said, it doesn't really matter how hard your champion scales into the late game that much. What matters is just pressuring the map from minute 1, which means even harassing the enemy jungler start. Pick something that can dominate lane (outplaying and getting advantages in low elo isn't even that hard anyway), and then just hard snowball from there on. Move around the map and DESTROY everything you see and take objectives, deny farm and just win the game.

Based on the above, the champion I'd pick is Jarvan. He is very easy to kill up early kills, especially at level 6. Go for a lethality build early on, and as soon as you get one kill, just push/take tower and then roam bot for free kills. Literally, his damage are also auto-targetted, apart from his Q, but once you get 2-3 kills, you don't even need your Q to one shot the ADC. Just E-Q onto them (if you do deal damage, then that's even better) and then all you need is an autoattack (passive + duskblade) and your ulti + Electrocute, if you missed the E-Q. This combo is pretty much guaranteed to hit and one shot the enemy carry. You can then proceed on doing this against the mid laner and pretty much everyone who doesn't have any tankiness. After a point, you will even be one shotting tankier enemies.

The game can snowball so hard this way. Every 80 seconds, you should be guaranteed to get a kill, which means probably a free push on a tower. After a point you will be easily picking up doubles and triples, which will make that process even easier. Eventually, you get to a point where the enemies will be afraid to even look at you and will either surrender, or have pretty much nothing to do.

Now, in case the game goes slightly weird, and you need some more tankiness you can also get some tank item, as long as you are able to 100-0 their carries. If you can do that, you can build some tankiness and just rush in everyone.