So.... you wanna be the most annoying character ever. Let's do it.

Flash is an OK jungler and laner early game, but amazing late game. I have the stolen powers set for jungling, but feel free to put on invulnerability or cameras for laning. My usual mods are upgraded marauder's ring, upgraded psi-scimitar, and reduced cost on helm of fate. The amplifiers I use revolve around cooldown per lvl, and and power dmg/lifesteal/penetration upgrades. 
When jungling flash, i usually don't use any stolen powers on the regen camp if i have a leash. I proceed to hit a triple camp and a quad camp with my stolen powers. I back and jungle based on my team's placement. 

When ganking, I use my q to get in, w for some dmg, then e stick to them.

Late game-- I use my q and w to target their carry, then ult when in the middle of all of them, and if they're not completely wrecked yet, i q through a wall and get the HELL out of there. It is also very fun to q into turrets, ult them under the turret, then q back out. With this build, you will lose will very easily, so back often, but try to hit the emps or regen camps before if you just made a successful offensive.

While laning, the late game is the same, but laning phase is really just hitting your q and w and running. Rinse and Repeat. When the enemy is low, ping and scream at your jungler until he is obese and cannot be fed anymore.

Have fun, Flash is probably my favorite character to play, and if hes not, he's DEFINITELY the most fun.