All games i start with:
Sapp crystal
mana/health pot

After I make a Rod of Ages because it gives health mana and AP which are all the stats ryze needs. It gives him good early game survivability damage and mana for overload damage increase.

I get my mercury's treads after my rod of ages. Kind of an obvious choice.

After I banshee veil which gives even more health and mana plus some magic res to go along with my treads. This gives ryze good defense against casters and absorbing spells is good. Mid game hes fairly difficult to kill and with loads of mana his overload hits hard enough.

I get Zhonya's Ring for the massive AP increase and mana increase. The 25% AP increase works good with his ult (granting an extra 37AP to the 150 granted).

Games that last long enough i would go Archangels staff for even more mana. By now I would have about 3400-3500 mana so the 2% grants about an xtra 75AP.

My total build is:
Rod of Ages
Mercury Treads
Banshee veil
Zhonya's Ring
Archangels Staff

Basically I start with mana/health with slight AP for early game overload/rune prisons for early kills and near the end i get way more AP since Spell Flux + Desperate Power becomes much more usually in team fights.

For summoner abilitys I usually do heal/exhaust or heal/teleport. I get heal because no1 else on my team usually gets it. Exhaust is nice to peel melee off myself or stop range from destroying me. Teleport is just kind of nice to have to bounce around the map, just a preference. I never found an awesome use for cleanse.

Feedback?

i like building tear of the goddess as first item. you are eventually going for archangels staff anyway and having a lot of mana midgame is great to kill with a low Q skill.

The last patch called, they want their AP Ryze build back.

Or is AP Ryze back in style? Last I heard I thought the new Ryze patch made the AP ratio's on Ryze no longer viable so now people played him as a high mana, low cooldown character? If I was going to go the AP route I'd go with Meja stacks primarily because if anyone can get those stacks it'd be a low cooldown spam-nuke Ryze and his shutdown and easy assisted kills.

Considering Overload does +10% of his max mana + a set amount of damage, if you had 3k mana thats 300 damage + whatever it would normally do?.

I tried the build out just now, we lost... but I kicked some ass. Very expensive items though, I didn't make it through the list of items to get. I skipped Rod of Ages cause I threw in a Glacial Shroud and Frozen Heart mid game. But it seems AP isn't totally out, but I go for mana first.