I start with a Manamune, as I tend to have mana trouble all game. Last-hitting with Parrrley is easier if you have plenty of mana to do it, and you always stay high enough on mana to help in fights or use your ult from afar. The boots you want are Boots of Lucidity, because it's really geat to use Parrrley more often and your ult. The 2nd main non-boots item I build is Sheen, for obvious reasons, but hold off on Trinity Force. I usually get Warmog's next so you're not quite so squishy. Then you can upgrade your Sheen or get Infinity Edge next, doesn't matter. Then do the other. Then get Phantom Dancer. Now, if you get extremely fed, late-game you can sell your Manamune to buy a 2nd Infinity Edge, giving you over 100% crit chance with elixirs. IE also gives slightly more AD than Manamune, and late game you won't need the mana as much.
Note: This is only for solo top, which I mostly do. If I jungle, I'll usually replace Manamune with Wriggle's because mana sustain isn't as important and Wriggle's is. Same rules apply; sell at end for a 2nd IE.
I wasn't really going to give my opinion/build on GP, but I saw that everyone else got voted down hard so I thought I would. :P I'm sure it'll get voted down, but meh. It works for me. To each his own.