For the most part, wizards treat the undead the same as they treat everything else; it’s not generally necessary to pack specific anti-undead spells, as whatever you use on everything else will probably work just as well against them. Whether the target is a brute or a mage is far more significant than if they are living or undead.
There are, of course, exceptions—enchanters, obviously, need a different approach. Necromancers, too, because they are more likely to have useful undead-specific options—and because many of their curses will fail against the undead. Enervation, for a major example, is out. But even in these cases, you don’t usually prepare something undead-specific, you prepare something that overcomes whatever failing your go-to spells have that render undead immune to them. Enchanters ensure they have non-mind-affecting options, necromancers have other forms of offense beyond enervation—and when chosen well, these things work against, say, mages touting mind blank or death ward as well as they do against the undead.
One of the most effective tactics for preparing to deal with foes who may be immune to your spells is to prepare buff spells to cast on your allies. For most parties, haste provides far more damage than any blasting spell, for example, and may well make short work of more magely undead. If forced to prepare your own direct-damage, orb of force from Spell Compendium is probably choice—and likely the best option for most wizards expecting incorporeal undead.
For direct offense, solid fog will always come to mind at the specified level, though incorporeal undead are immune and magely undead may have freedom of movement or teleportation available (in the latter case, forcing them to use those options can still have value, though). Black tentacles, as mentioned, is also strong here, but has the same weaknesses. A perhaps better option, especially if incorporeal undead are expected, might be resilient sphere—but it relies on the wizard having not prohibited the evocation school, which most wizards do (since contingency and resilient sphere are about the only evocations of substantial interest to most wizards, and shadow evocation exists, arguably, for the latter).
If an undead-specific answer truly is desired, command undead is a serious spell. It lasts for days, mindless undead get no save at all, and while it takes Charisma to order around intelligent undead, even if you fail, they can’t attack you and treat everything you say in the most favorable possible light.
With the exception of orb of force, all of these spells are core. There may well be even better options in the books; I didn’t check. I think it’s telling that core alone offers all you really need and then some, so I think I’ll leave it at that.
