So i know this isn't the best, but it works very well. i go legendary very frequently with this build, so i don't ill change anything around much. you can vote on it if you want, i don't care, i only use mobafire to link people a build i make rather than talk it all out to them.

i use weird masteries bc i like mobility alot, and with my runes i start off not needing boots for awhile and end the game moving nearly at teemo speeds

i get a harpoon first for poking/last hitting purposes as well as brush checking(if sparks go off after you launch it connected to an enemy). then max Q and pick up a W & E every over time.

k i open up with tome and pots

build into rylai's picking up either belt(if having troubles) or blasting rod(if you're giving them trouble) on an early trip back for wards.

the spell pen boots if you're doing well otherwise merc treads usually

then a sheen

if team is doing very well build into lichbane and completely follow the shown build and remember to make use of gunblade's active while overheated

if team is lacking or you're the target of all the enemy attacks build sunfire and scepter early then hextech revolver then go back to shown build.

works very well in nearly all situations. if enemy team is REALLY AP heavy get a FoN or QSS sash instead of sunfire
If REALLY AD heavy thornmail maybe

and probably the most important part is have fun. the fat announcer with long hair at dreamhack always referred to rumble as "Ze Rumble" so i talk in a funny accept about him while i play. and i like imagining overheat is PWNING PWNING PWNING rather than warning
Zey no can catch ze Rumble!