I'd automatically disqualify anything made in the last century or more. That whole 7 world wonders thing was a farce anyway. If your country cared to run a nationalistic vote in campaign you were a finalist, if they didn't, you typically weren't.

Hagia Sophia is absolutely #1 on my list.
St. Basil's Cathedral for. Nothing like it that I know of. Just an incredibly unique structure.
Angkor Wat is another one of those that stands out, must see.
Always been curious about Petra in Jordan.
Potala Palace is badass
Sagrada Familia may not meet my initial criteria, but fuck it. Its an awesome, unique structure.
Gotta see it.
Tell me it isn't a bit nuts:

I'm sure there are some others. Bunch of shit to see before I die.