I've mistakenly cast a vote for "all of them", I don't own them now but I plan to own all three at some point, at least after I get a 360 just for the sake of owning one. It's nice to have each one to have a taste of all the gaming generation has to offer - it's always been my policy. I may have my favorites, sure, but each system has its very own unique qualities making it a worthy purchase.
So yes, I'd like to ultimately own all of them, but currently I own none since launch consoles are just a tad too expensive for my taste - I'll wait till the prices drop and until then, I'll carry on filling in the gaps in my collection.