Battlefront, both of them but honourable mentions to both Kotor. This was actually a tough one to pick. I loved both games. I really loved Kotor and I completed it that many times it's almost unbelievable. Heck I completed KOTOR 2 enough times that I could have written a walkthrough. I turned every crew member possible into a Jedi and this was before I owned a PC and before I could look up how to do it.
However Battlefront really hit it for me. There's not a game out there that I've completed more times. Not a game out there I've spent more hours on. Just the ability to play a red shirt or a nobody in the big star wars battles was a brilliant and intense feeling. The ability to pilot an AT.AT and just destroy the majority of Hoth. The dog fighting battles you could do in the second... Taking out a star destroyer from the inside out or going on a suicide run and taking out the bridge and other important magufin.
I also loved the maps you got to play in. Mainly because they where iconic maps from the movies, like the Death Star, Tantive VI, Hoth, The moon of endor, Degobah and even Jabbas Palace. You felt like your a part of the iconic moments in Star Wars. You was faceless, you weren't Luke or Han you were a faceless mook to which you could project your self to. Granted the sequel did ruin it by adding the control of heroes however I can over look that part.
I also liked the clever little homages made in the maps like the garbage crusher on the Death Star. That was cool. I do hope they make a sequel but I doubt it's going to happen. With the sounds of things the company went bust or something like that.