Usualy the best picks is Support and AD carry as first 2 picks, then jungler, then mid and then top. Top is usualy the place where the most counterpicking is so try make pick top as lastpick. Unless you can get one of those perma-banned champs like shen/morg/shaco since they all got very safe lane phases.
As Drmixable mentioned, it is very situational. If you're intending to solely be a kill lane, it might make sense to go Sapphire Crystal, ward, health pot, then tears.If you do this, you are gambling completely on your lane getting first blood and then compounding that advantage into laning dominance for the rest of the laning phase.
If you are wanting to be a more traditional support early game, then definitely do the standard Faerie Charm, wards, health pots mix.
Even if you do start with the mana crystal and go straight into tears, I think it still makes sense to move towards the more traditional support build after you pick up the tears. Philo stone, Heart of Gold, lots and lots of wards, oracles, etc. Build your gp10 items into their upgrades, prioritizing Shurelya's if your team is winning team fights and Locket if they are not.
And - as always - buy boots on your second trip back to base. For Blitzcrank, Boots of Mobility upgrade later makes a lot of sense for getting into position to yoink someone or knock them up to protect your carry. However, if the team is full of hard CC you will of course want the tenacity from merc treads instead.
Other good items include getting an early catalyst(which will build into Banshee's later), to sustain your lane harass and increase your passive shield proc if necessary. An aegis, of course, is always welcome from a support. Frozen Heart could be viable if the enemy team has more than two sources of straight auto attack damage, but generally won't be worth it.
Hopefully this wall of text gets you started on the path to thinking about how to apply your items to each situation. There is no single item guide that any hero should follow. Anything from who you are facing, how much gold you have, and how late it is in the game can vary your items and the order you build them in.
GL HF!
Best Answer
You can gave movement speed through 2 master abilities: Initiator (3%) and Swiftness (4%). Also, you can take Summoner's Wrath to improve your Sprint speed by 35%.
Obviously take Ghost as your summoner.
You can rune 3 movement speed quintessences for an addition 4.5% move speed.
Boots of Mobility offer the largest boost, of +5 when out of combat. Add in 5 Phantom Dancers for an additional 60% movespeed.
Finally, while it's not sustained, Rammus' Powerball move reaches the highest speed of any champion. For sustained burst speed, such as across the map, Master Yi's Higlander is better.