Alone.
The only way I'd ever ctb with someone else is if they've become so close to me that they managed to open me up completely.
But if I had someone like that, I'd probably want to be around for them (read: I'd be absolutely obsessed with them). I'd stick around until they had enough with life and wanted to go too (which may never happen), then at that time it would be a partner suicide instead.
Buuuut... My walls are too thick, and no one wants to deal with that, so alone I go!