Doesn't have to be just one or two, name them all
In order of most favorite
ME1
Male: Garrus, Wrex, Joker
Female: Tali
ME2
Male: Garrus, Thane, Joker
Female: Kasumi (Master Thief DLC, shame you can't romance her), Tali, Samara
I'm really looking forward to what they'll be doing in ME3, truly can't wait!
In order of most favorite
ME1
Male: Garrus, Wrex, Joker
Female: Tali
ME2
Male: Garrus, Thane, Joker
Female: Kasumi (Master Thief DLC, shame you can't romance her), Tali, Samara
I'm really looking forward to what they'll be doing in ME3, truly can't wait!