Not Updated For Current Season
The trick to winning Twisted Treeline is playing off-tank assassins that easily get kills (early game), and making the enemy surrender (one way). It's always smart to start with boots as survivability is pointless, because your base is always so close. Next, you'll need cheap items that are yet effective, like Tabi, Mejai's, Wriggle's and revolver.
When it comes to summoner spells, flash is useless because of it's long cooldown...you barely get to use it. Offensive spells are much smart so you could get the masteries for them in just one point. Also, you should also get 21 points in offense, and the health mastery (or mana...)
Back to my point with cheap, effective items, you should stack Doran's after boots, and getting a Wriggle's along the way is really useless...since there's only really two placed to put it (lizard and dragon). Mias are really obvious in Treeline, but many people like getting a few monsters when they have low hp, before they recall...and you could easily spot them out this way. Now, you should build for Trinity Force, as the items in the build path are cheap, but of course, you actually have to make use of the sheen proc (no matter how little, like Shaco and his q, AD champions with low cooldowns are also encourage to get tri-force). The next type of build would be HP + Atma's, quite effective on Darius and Olaf, you could also stack Doran's along the way.
Once again, boots first, questions later (and an optional mana pot). Kage and Mejai are a must, with optional Doran's Rings along the way (unless you have mana masteries). Since you don't want to get cheap Tabis, just don't buy tier 2 boots at all, you could live with ghost (you could buy once you got all your cheap items down). For hp, you must get revolver and not rod, as you probably won't even get to level 18/full stacks before the game ends. If you use a lot of mana, consider Athene's.
Some defense items, such as Banshee's and Maw, are overwhelmingly expensive, and quite useless too. Maw is for champions that are usually not at full hp...not likely in TT, where the base is close and level up is easy. As for Banshee, basically everybody saves their ult for the team fight, you'll get your Banshee popped almost immediately.