Easiest junglers to learn and do well with; warwick, fiddlesticks. both have near infinite sustain in the jungle.
I'll also add AD yi to that list. with the buffs to base heal on meditate he can farm the jungle extremely well and is fast enough to counter jungle in his regular path. his highlander ganks are also hilarious.
for learning mid, its a tough call cause mid is in flux ATM, lots of AD mid cropping up to stomp low levels (think panth, talon, khaz, etc). so I don't have recommendations there.
for top lane, garen is easy as hell and you just have to avoid getting countered by a teemo.
don't very familiar with bot lane, but taric is an easy, versatile support to use while learning.