Any number of the churches/cathedrals I've built.
They're not entirely realistic, and probably not attractive to most, but I've always been fond of them.
Plus, I usually hate building them, and find it a drag to do so, but once they're complete, I find myself quite proud of pulling it off.
If I had to choose my favourites, I'd have to say either a nice large cathedral-ish one, which I built for a medieval town, overlooking a lake, or a reasonably large one with nice stain-glass platebrick windows, which I built for an application into Real_Neo's EC Clan.
I mainly applied because a friend was in it, and wanted me to try and get in.
Not much happened with it, but the people who were in the clan were really nice people.