I ship, oh, let's see. Archer/Issei, Archer/Rin, Hakuba/Issei, Issei/Haruto (a pairing where Issei tops? Say it isn't so!), Ban/Rider, Kaito/Aoko, Kaito/Karen, Hakuba/Akako, and Kuzuki/Caster, among probably tons of others I forgot about.

Issei ships Kuzuki/Caster and normalcy/Haruto. Sadly, both of those seem to be sunk ships.

Issei's crushes right now are one on Shirou, one on Archer, and a growing one on Haruto.