Generally speaking, I always take Meki pendant and 2 mana pots. Get sapling first. Then I go...
Chalice->boots->start on catalyst-> Merc treads->RoA.
After that, I start looking at team comp and my team. If I have a team that typically dives in and gathers, I get Aegis. If it's a gank fest, I get HoG. Philo Stone is good too. Late game, my build always varies, but excluding RoA and Merc treads...
AD team- Randuin, SFC, possibly Thornmail.
AP Team- FoN, BV, possibly Abyssal Sceptre
Mixed team, low CC- Aegis, GA, Randuin
Heavy CC- BV, QSS, SFC
All and all, the core items, imho, should be RoA and Aegis. Dump the chalice once you have a good amount of mana. Oh, and NEVER skill up your ult after 6 till you are forced. Waste of mana.