If I'm solo mid (i.e. carrying), I rush Zhonya's. I know it's passive isn't "cost effective" until you have some other AP items, but you can use its active during your ult, making it absolutely priceless. Afterwards, I'll build to the situation at hand, but typically you want to focus on AP, CDR, survivability and mana. Personally, I like Glacial Shroud, Haunting Guise, and Abyssal Scepter. Archangel's, Rylai's, Mejai's (unless you're having a really bad game, you'll get plenty of kills, and even more assists) and RoA aren't bad choices either. Also, don't forget about red and blue elixirs (I'll often grab a blue one right after I pick up Zhonya's). For summoner spells, I take ghost and ignite when I'm mid. Flash is a decent choice too, but ghost is more reliable IMO, and that extra oomph from ignite really makes the difference if your spell combo doesn't quite get the kill.
If I'm stuck in a side lane (i.e. supporting), I usually go RoA -> Mejai's (optional: don't take if you're having a bad game) -> Zhonya's. Since you're lower level than you would be if you solo'ed, you have quite a bit less health, and thus RoA is a better choice than Zhonya's IMO. Also, since you're in more of a support role, I replace ignite with teleport, so I can quickly get to where I need to be.
TL;DR- If you're solo mid, rush Zhonya's. If you're with a partner, go RoA first.