First playthrough (FemShep) I didn't have the luxury to "choose" anyone, because Garrus and Jacob didn't appeal to me, none of the girls swung that way, and I did Thane's loyalty mission way too late to have a chance of getting some there. Shepard ended up getting together with Kelly after the suicide mission was over, so we'll see what Kaidan's ego thinks of that in ME3. =P
Second playthrough (MaleShep) I chose to go after Jack from the get go, and have no regrets. Felt a bit sorry for Tali, though, who was all ready to confess her feelings to Shep. Also looking forward to Ashley's reaction in ME3. (How I Met Your Mother totally ruined Ash for me. =( )
Third, current, playthrough (FemShep), I'm intentionally staying loyal to Liara, since I come prepared with Lair of the Shadow Broker now. Not done yet, though, and I hope I didn't accidentally flirt with Jacob earlier. (BUT THE PRIIIIIIZE...) >_>