best top imo. High mobility, high damage, high sustain, no mana, has a stun and heal, awesome wave clear, awesome harass, tanky. I also take him jg when i'm forced to jg..
best mid imo. Very high mobility, very high burst damage, great to gank with if you roam, can dodge turret shots so kills are easy. Plus you will never get creep locked since his passive lets you move through units and his DoT is a free ignite.
best adc imo. Best range in the game, can be built in many ways suiting your preferences. Passive guarantees if you die at least you can take one with you. Can always get enemy adc hiding under turret with ult
Best support imo. Permanent slow to enemies or speed buffs to allies. Has a stun and great zoning capapbility. Does your adc like to turret dive? slap the with your ult and the turret dive double kill is no problem
best jg imo. No mana, high sustain through passive, AOE damage for camp clear, easy ganks even over the thickest walls, tanky, high utility with 2 knockups aoe damage and a slow that can hit multiple targets, and he is super FUN to play!
runner ups
top/mid, very underestimated
top/jg/mid, very fun to play, good ganks
mid/adc, the global pressure and easy ganks with w if you can handle his low health
support or "adc" ult fares better in bot lane, fun to play, high damage and 2 sec stun
top/support. The definition of a lock-down champion
top/adc. fun to play, fares well top since most tops are not ranged
if you need help building any of these champs just ask, but you are on the right path. I think everyone should be at least decent at every role. Good luck man hope this helped a bit