Personally, my favourite jungler <=4800 is
and I find he's the one I have the most success carrying games with. If you know how to play him, farm efficiently and pressure lane then you can carry rather easily with him ever after the nerfs.
My favourite mid-laner at the moment is
due to her safety, utility, poke, waveclear and ability to catch targets. Previously, it was Ziggs due to the same qualities (with less emphasis on utility and more on waveclear/poke.
For junglers, I would recommend champions with a good mixture of durability, damage and CC. In my own experiences in Silver, many players would pick low-CC damage-oriented champions without regard for team composition and the very fact that your team had a durable champion and CC would be invaluable. I'd recommend the following:

For mid-laners, I would recommend picking champions with safe waveclear and CC since many lower ELO games tend to devolve into ARAM. I'd probably recommend the following:

Even if you fall behind, you can stall the game out by wave-clearing and you have the ability to engage/disengage fights safely.