Fart n' Dart is a Tank/AP hybrid build for Singed that focuses on making use of his excellent mobility and survivability to both draw the attention of the enemy team and do substantial damage to them at the same time. Rabadon's and Mejai's combine together to give you insane AP if you can keep your stacks, and Force of Nature and Guardian Angel make Singed, who's hard to bring down as it is, nearly unkillable.

This build works very well for 3v3 but I have not done extensive testing with it in 5v5, regardless I think it is safe to say that Singed works better with a lane partner then in solo, this build especially.

Greater Quintessence of Swiftness

Move speed is critical for Singed. It allows him to initiate with Fling easier, lay his Poison Trail across a larger area, and gives his much better survivability and chase potential. Early game, with move speed quints (as well as the masteries), you will move nearly as fast as someone with basic boots. This fact alone can allow you to dominate most lanes with Poison Trail.

Greater Glyph of Intellect

Spamming Poison Trail early on in lanes can hurt your mana. These runes help with Singed's early game mana problems while giving him a little bonus health in the meantime with his passive, Enpowered Bulwark.

Greater Seal of Evasion

This is the best rune you can take if you're playing a tank. Helps your survivability a ton and synergies with the Nimbleness mastery to give you a bonus boost of speed in a messy situation.

Greater Mark of Insight

I choose Magic Pen in runes because I'm not picking up any in my items or masteries. Any AP character serious about dealing the most damage they can should have at least one source of Magic Pen (be it in items, runes, or masteries) to combat the enemy's natural MR, and should take more if they are stacking MR in items.

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Defense

These are basic masteries to increase Armor and MR. I choose increased health regen over blocked physical damage to synergize with my ultimate and with Force of Nature, and extra dodge change and Nimbleness for excellent survivability and escape. Lastly I take improved Clence, since you are so much more vulnerable when Clence is on CD.

Utility

These Masteries Focus on health and mana regen, for strong laning presence early on and good synergy with your ulti later on. Other Key masteries include improved Ghost, and the move speed bonus, both are critical to this build's effectiveness. Greed isn't really necessary, but I had an extra point I needed to spend in Utility, and who doesn't like a little free gold?

Starting Items

Sapphire Crystal and Health Potion x 2

I've found these starting items to do the trick in pretty much any situation. The extra mana lets you throw your poison around a lot more early on and gives you some bonus health.