[Original Response to Subject before reading the post, thinking you meant "what should a wiz have in his spellbook?"]
Identify
All the other spells are subject to a Wizard's personal style, his specialty, and his theme. Identify however is such a universal spell that no self respecting Wizard who is out there to study the world (Wizards study a lot, right?) would lack the easiest dweomer to discern the properties of a magical item.
[Response after reading the original post]
The most essential thing to have in a 1st level spell slot is... Nothing!
I'm of the opinion that Wizards should always be able to provide utility at the cost of a little prep time. If you want on-the-spot spells, you need a Sorc. The open spell slot is the most useful thing to have because it and 15 minutes gives you access to all of your spells. Even a 3rd level mage should have one 1st level spell slot left open at all times since he isn't designed for blasting things, but for trouble-shooting. The spells in the other two 1st level spell slots will vary, but there should always be a slot open for whatever the circumstance require.