Some that come to mind:
Pikachu & Seviper, since I collect both of them. 
Magikarp & Feebas
Milotic & Cresselia
Seviper, Arbok, and Sandaconda (since snakes are my favorite animals)
Here two similar earlier threads with a lot of good (and some disturbing, haha) suggestions:
www.elitefourum.com/t/what-is-your-dream-tag-team-gx/23554/1
www.elitefourum.com/t/if-you-could-make-your-own-team-up-card/22872/1
Greetz,
Quuador