Hey I'm Icon777 currently Silver on the NA server. I have an average KDA of 9.5/6.6/9 on Rumble across more than 120 games. Let me show you what I've learned

You should be rushing seekers armguard in a matchup against an AD bully top laner. Otherwise and especially against tanks rush Haunting Guise for the Pen and then get lucidity boots. Zhonyas is next after the because of its synergy with your (Q) because it can be on while in stasis. After the first 3 are completed it's really situational, is you're carrying get rabadons, if behind get rylais and if late game against lots of MR get void staff

Flamespitter (Q) is maxed first for its massive base damage especially when paired with your passive. Electro Harpoon is ranked second because the slow gets absurd once you get a few points in it.

Flash (Duh)

Telerport - Pick almost 99% of the time you will snowball harder with this than ignite because you can tp bot for free double kills and lategame you can instantly join teamfights and AOE down the enemy team.

Ignite - Please dont take this summoner spell. Ok you can take it against Teemo if you want to make him cry (or many other favored matchups). Also ok if you're against a high sustain top laner such as Vladimir and your mid laner didn't take Ignite.

Not much to say here, Rumble likes to get up close in people's faces so defensive runes are a necessity. AP of course buffs his scaling and as mentioned before Rumble is very strong with magic penetration especially because of his high amount of area of effect damage.

In the ferocity Tree, AP and magic pen are very crucial. Magic Penetration is the best thing you can ask for on Rumble because it buffs the extremely high base damage from his Flamespitter (Q) and works well his his two AOE spells.

Although the Resolve tree may seem boring because it doesn't give you more damage to melt the enemy team with, it's very important to have on Rumble. Since you'll always be burning people up-close you'll need some defense in-case they try and fight back. Unyielding is good as it buffs the armor from Zhonyas and buffs the MR and armor from your runes. The absolute best mastery from this tree is runic armor that when maxed out will add 8% bonus shields to your Scrap Shield (W)