jungler is much harder, not only do you have to keep your eyes on 5 parts of the map at once or more, the monsters you are facing can kill you depending upon your role and you have to constantly adjust your position while having an uneven flow of experience and gold
was just in a match where both junglers were level 7 while the enemy team was up to 11
this is why junglers like reksai and pantheon are more viable, but I think pantheon makes a better top because of his all in capability, but it's really dependent upon the matchups
overally, I think the list of difficult from least to greatest is Mage, ADC, support, tank, jungler, but tank and jungler are fairly even and dependent upon matchups as well
some people would argue that support is the most difficult role because they have to pay the most attention to their lane and their items, but most supports are point and click while they're at that. Ex: blitzcrank, soraka, thresh, nautilus, sona, chogath, annie, teemo, anivia(good thing riot is reworking her soon I think from another post), janna
pretty much all supports, that's why they have to pay attention elsewhere because their kits are easy to use since they're not supposed to make fancy plays except for thresh and bard
Bard is one of the hardest supports at the moment because that's literally all that he is. he has little damage, little crowd control, small heals, and his scaling passive is leagues behind nasus/veigar/sion. What he can do is allow a tower dive to happen immediately at level 6 though and he allows champs without an escape an escape mechanic
compare bard's chunk damage to someone like soraka. she can easily do 50% of a carry's hp early game in 3-4 spells while healign her carry to full
sona does nearly the same thing with a bit less burst damage/heals and less crowd control
zilean does a very large amount of burst damage for a support and can keep a carry/himself from feeding kills (if he's lucky XD ) (however, I think zilean should keep to his mid-lane role because when he's support you have no heal options and little crowd control unless the enemy gets hit by chance (his bomb's sticky radius is also a little glitchy, but not enough to notice)
now, the list of ADC's goes on and on, but most ADC's are free to move as they please and have a variety of poke abilities or burst abiilties
take draven for example. his range is a little low except for his ultimate, but he has weak crowd control, a move speed steroid, a damage boost, gold increase passive, and a global ultimate
which ADC do you have trouble with? Ashe? she's one of the longest ranged champs and permaslows her victims. Sivir? they really didn't need to nerf her, come on...with W and runaan's hurricane, she can pretty much instantly nuke your entire team, that's why her range is so low
there are too many to list, but in the end, it is your personal opinion