Here's my Top 5.
Green Arrow. So similar yet so different.
Nightwing. Bruce and Dick are the best duo in all comics.
Zatanna. I like Batman in supernatural stories and I dig the flirty thing they have going on .
The Creeper. I just dig the two of them together. The "Good Joker" should be a regular Gotham character.
Plastic Man. Always a fun team-up.