Seconded.Thanee said:The wizard.
The only way a well-played core LV 20 Monk would win against a well-played core LV 20 Wizard is is the Monk could take the Wizard out before the Wizard got a chance to act (keeping the Wizard stunned is a theoretical way to do so). Barring sneaking up on the Wizard and coup-de-gracing them in their sleep (which a competant wizard could also likely avoid if desired), this is unlikely to happen.
Mind you, at very low levels, I'd favor the Monk, but here are how a few strategies would likely work:
1) Grapple the Wizard:
Wizard: Teleport away (say, home, or up 1/4 mile) come back a few rounds later (or 1 with time stop) fully spelled up. Blow the Monk away (unless the Monk is smart, and chose to hide really well).
2) Take the Wizard out round 1:
Wizard: if you don't have the HP to take a single large hit from a Monk, you're unlikely to have lived long enough to hit LV 20. Later in the round, blow the Monk away.
3) Steal the spell component bag:
Wizard: Teleport to a major city. Buy another one. Teleport back, this time fully spelled up. A few minutes later, blow the Monk away (unless the Monk is smart, and chose to hide really well).
4) Stun, round 1. Continually stun until dead:
Wizard: Actually, this has the largest chance of working. The Wizard's fort save should give them a decent chance of saving, but well under 100%. The Wizard might not have a protection against this, and the Monk is likely to pierce any DR. A decent Wizard build by this level should have around +15-+16 Fort Save. The Fort save should be in the range of 24-26. this gives the Monk around a 48.5% chance of stunning (best case), and they should be able to pound on the Wizard until the contingency intervenes, the familiar intervenes, or the Wizard dies (assuing that the Wizard continues to fail saves every round).
Caveat: I've never played a Wizard at this level (or a Monk) so I may be missing something. I have, however, played a Sorcerer at this level, who would essentially do the exact same things I described above.