Ottawa.
Now, I can pronounce it two ways, actually:
1. Ottawa (with the t)
2. Oddawa
The second is the way people expect it to be said. The first is the way I would prefer to say it because I happen to have learnt it that way. When people ask me where I'm from, there's always an internal struggle between Ottawa and Oddawa before something mangled comes out.
If I do say Ottawa, people always ask where, or repeat "Oshawa"? which is another Ontario city. Then I have to say Oddawa. No one expects the ts in Ottawa.
If I say Oddawa, chances are it'll be so mangled that I'll end up repeating it anyway before someone graciously translates and saves me from being more embarrassed than I already am!
It's a loosing battle and I think saying "the nation's capital" would be a little pretentious.