I'll agree that Greater Invisibility and Dimension Door are about as close to essential spells as you can get. Greater Invisibility wins by a huge margin over Dim. Door, I think, but I doubt there's a single wizard out there who wouldn't want both of them in his book.
I also eventually gave the nod to Scrying and Evard's Black Tentacles, though I'm not as enthusiastic about those selections. Scrying's just too handy and too valuable in higher-level campaigns to pass up, I think; if you take it, you will use it, and if that's not the definition of a must-have spell, I don't know what is.
Evard's Black Tentacles is also an impressive spell, although whether it will actually be useful to a wizard depends greatly on the style of the campaign. In our game, for example, I opted to not get it because we tend to run our combats very freeform (no mat, just cheesy GM maps of the area we're in, scrawled out on notepaper); trying to figure out where the tentacles would end up and how to use them would just be too much of a pain in the ass to be worthwhile. But if you're in the mat & miniatures camp (or even just more inclined to spend time figuring out things like that), well, Tentacles will always be a winner for you.
A wall spell is also a good backup choice: I like Wall of Ice better than Wall of Fire, personally: it has a better duration, it's more flexible, and it does a better job of blocking opponents. But obviously, opinions might differ on that. And finally, if your characters are ridiculously wealthy, stoneskin is a great spell; it's just a poor choice if your character can't afford to keep the material component on hand in large quantities, because if you're noticing how much it costs per casting, you'll either regret it every time you cast it, or (more likely) you'll find that you never cast it at all. It's enough of a limitation to keep it out of my first picks, anyway.
Actually, when we were playing, I typically memorized a couple of spells out of Relics and Rituals 2; it has a much nastier 4th-level selection than you'll find in the core rules.
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i'd usually have affliction and purifying flames racked
ryan