I tend no to let my opponants magic phase try to bother me too much, but with my TK, the following SHOULD be the most destructive:
- Tzeentch Mortal
- Slann
- Teclis.
However, i have not yet played vs Teclis (That game is lined up for easter) I have never faced a Slann so im not sure exactly how that would turn out. And, well, the two times i have faced a Tzeentch list,
1) 600pt Lord miscast, ended up in combat with some skellies. Lord flufed it, skellies ran him down with a persuit

... And
2) The Tze guy got 2 spells off all game! well, 3, but he tried casting the one that doesnt affect ItP. Still, the fact you need to roll to cast hurt him a lot.
Bassically both times Ive played Tze, ive massacred.
The army i do find THE hardest to face is Skaven at low points, i.e ess than 2K, is they have 3 Engineers with WL and one has the bound spell. That is some nasty magic to face.