Not Updated For Current Season
4/27/2011: So, Rumble just came out and I haven't played him myself but this is a quick rough build I'll probably edit once I get to playing around with him. The point of this build is to get him to be bulky and at the same time dealing damage as much as he can.
Note: Please feel free to make any changes or suggestions since I have yet to gain any experience with him, thank you!
[ Mark/Reds ]
- Greater Mark of Insight (+0.95 Magic Penetration) x 9
[ Seal/Yellows ]
- Greater Seal of Defense (+0.15 Armor per level) x 9
[ Glyph/Blues ]
- Greater Glyph of Celerity (-0.05% cooldowns per level) x 9
[ Quintessence ]
- Greater Quintessence of Fortitude (+26 health) x 3
Magic Penetration reds for more damage, armor yellows for defense against physical attack,
cooldown reduction blues for spamming spells, and flat health quints for bulkiness.
Start off with Amplifying Tome so that you can harass early on and get early kills, then build Ionian Boots of Lucidity so that you can spam your abilities more often (If there is too much CC on the other team, you may want to consider Mercury Treads instead). Then work your way to build Rylai's Crystal Scepter which will give you AP, health and the ability to slow your opponents.
To make Rumble more bulky, I would start building Warmog's Armor so that he can get tons of HP making it difficult to take him down quicker. Especially since he can farm well, he'll be able to gain HP on Warmog's Armor quickly and farm gold for his AP items.
After that Rumble should start increasing his damage output. Start building Rabadon's Deathcap for that raw AP that gives casters a boost in their damage. From here, you can start building Zhonya's Hourglass or Abyssal Scepter depending on if the other team is caster heavy or physical heavy. If the other team is pretty much even, start working on Abyssal Scepter since it's cheaper and will help your teammates out (and yourself).
Can't say much about this since I have not played him first hand but starting off with Q sounds like a good choice whether you are in the middle or laning with a partner.
[ Goal ]
The endpoint of this build is to have Rumble handling some off-tanking action in fights and dish out damage without getting taken down in a matter of a few seconds.
It is notable that while the cooldown reduction isn't high in total (about -26.1% cooldown reduction), it will still help him in dishing out damage over time.