Well here's the thing most champions that would be capable of carrying dead weight are typically not super easy to pick up, you might be able to understand them in 1-2 games, but actually mastering a champion is where the climbing ability comes from when maining a champion. So while you might understand them quickly, you'll need to realize that it may take several games to really develop the ability to carry your teams with such a champion, and I don't mean several as in 6, I mean several as in 30 depending on the learning curve of the champion.
That all being said the champions I'd recommend, given the requirement is top/mid are Gangplank, Pantheon, Kennen, Fizz, Ekko, Jayce, and Lissandra
Going down the list.
Gangplank is better top than mid because he struggles against ranged matchups, but he's a champion you can fail at in lane and still become a complete monster late game. I had a game where I got ganked nonstop recently, ended up 0/3/0 by 10 minutes, but after that I started scaling, and at the end of the game ended 15/8/19, with over 40k damage to champions, and we ended up winning the game carrying our 5/14 lucian and a jungler who was 0/8 up until the 25 minute mark. Gangplank is actually the champion I've decided to main recently, and while it was frustrating at first to do well individually and not be capable of carrying the team in some games, I learned it was primarily a situation where I wasn't applying my strengths right, which was only something I could learn by playing several games with the same problems happening.
Pantheon is more of a cheesy pick, you choose him to pummel lane very quickly, and then drop that advantage on other lanes, literally. He's not good against everything, and can be abused quite easily.
Kennen is pretty solid, only issue is he largely carries through team fighting CC, so if your team doesn't wish to group up, as some don't, you're not going to be very effective.
Fizz and Ekko are still both the same snowbally nightmares they always have been, both also have variable builds you can try out, be it full AP, AP bruiser, Triforce with Tank, etc.
Jayce is an interesting pick who I feel is similar to Gangplank in that he takes a lot of effort to learn him, but when you do he's exceptionally strong. I used to play a fair bit of Jayce and always had fun with him. He's a lot easier than Gangplank in my opinion, and his turret pushing is as good if not better due to hyper drive. His early poke can be very oppressive, and his all in potential is massive all lane long. The main thing is that you need to learn when you can and can't go in, as his damage can be very enticing, but the last thing you want to do is jump in before all their CC has been used up.
Lissandra is pretty solid, she can be abused quite easily, but works well as a lane bully herself in some situations, she also has her ultimate, which comes with the added benefit of being nearly a guaranteed kill on whatever target you choose to use it on so long as your team follows up. She also has a similar problem to Jayce in that you need to be careful to not just jump in automatically.
I would actually recommend all of these champions aside from Lissandra to be played top over mid if you have the option I also suggest trying to take top more frequently than mid lane as counting on getting mid for your climb is kind of like gambling to pay your bills, you might get it sometimes, but more often than not you're going to get fucked over. This also has the added benefit of removing the likely potential of your top laner getting obliterated as usually top lane goes one of two ways. Either your top laner gets annihilated, or their top laner gets annihilated. It's very rare for them to come out about even.
