Definitely do We Love Sundays @ Space IMO the best day/night on the island, followed cloely by DC10 on a Monday morning.
Other than that it really depends on what you are into.
If you've never been to Ibiza before it is worth going to Manumission just to experience it, there are better things to do on a Monday and I wouldn't personally bother with Manumission these days but the first time you go and experience the sheer vastness of the place and then the sun coming up over the roof is unbelievable.
Tues (Perfecto), Weds (Subliminal) and Fridays (Ministry) @ Pacha are all must-dos for me, but you might want to venture to more different clubs.
Cream @ Amnesia is good if you like trance, but they normally pack it out so much I haven't been for the last couple of years.
Re. pre-drinks I might pop down to Mambos if the pre-party is going on there, but to be honest the pre-parties aren't all that, you might get one of the DJs who is playing later on at the club play for about 30 mins if you're lucky
I like to have some drinks down at Bora Bora before I go out, guaranteed to get me in the mood for clubbing
or otherwise Base Bar, Rock Bar, Kube in Ibiza Town if I'm going to Pacha cos then it's just a short walk to the club.