If you can only go to one club---make it Space!
Having just returned to Cali after spending a week in Ibiza (including the Sundays at Space opening) I have to say Space is not to be missed. We also went to the Cocoon opening party at Amnesia (VIP, Sven Vath etc.

), and the Pacha Underwater Party. On our way home we got to see Digweed live at Deep in Madrid.
I don't do drugs and that is one of the really great things about Sundays @ Space. It is going on from 8am to 6 am. We went for the morning (something we never have the opportunity to do in the US!!!) and had a blast. We went to the beach, took a siesta and returned for the evening. I had a difficult time enjoying Pacha and Amniesia for very long because by the time they really get going (about 4-6 am!!!) I can't stay awake any longer. I had to leave in the middle of Sven Vath's and Digweed's sets

because I was sooo tired.
The terrace at Space is THE place to be as far as I am concerned. Ideally you should experienece it in the morning AND at night. It has a very warm, freindly vibe--perhaps it is because 1/2 the people are on X but it didn't bother me

. Find a place in front of the fans and get up and dance

And don't miss Lawler's set--he rocks the Terrace!!!
Pacha is very Posh. It has lots of little cozy places to chill and a couple of different floors with different music (hip hop, in addition to house for example). Very classy establishment, convienient to the harbor. Gets going around 4 am--don't show up before 2 am.
Amniesa is big, loud (your body will vibrate and your hair will stand on end!!!), and does lots of great effects (lasers, go-go dancers and other performers in costume, shots of icy cold air up your skirt (if you are on the main floor).
Have fun!