If you plan on getting them all eventually I'd get them in the order they were released. That way as you aquire more expansions it won't be a pain in the butt to install them (they have to be install in the order they were released). So...if you do want to get all of them...I strongly recommend you get University first. Then Nightlife, then OFB and then Pets. Although personally Pets is my favourite, followed by Nightlife and OFB. Cass.
I second Cassiepeia's suggestion...Each of the expansion packs add something significant to the game play, and it is a ROYAL PAIN if you add a later EP first, and then have to uninstall it so that you can add one of the earlier EPs.
