1 set the standard, that cannot be argued. It was gorgeous, immersive, and fun. However, apart from the light/dark aspect, everything was all the same through every playthrough, and you could abuse your companions as much or as little as you wanted without consequence or the story changing.
2 would have one-upped it, were it released in its completed form. But as it is, the atmosphere is better, and gives you a feeling that what you do actually IS making the difference- almost everyone takes the time to remind you "The galaxy is in shambles!" and deliver their personal view on what you've been doing- the companions are much more interesting, HK-47 seems more psychopathic than before, new abilities to play around with, and most importantly, I didn't have to wait until half an hour to midnight to get some nice loot.
And Brianna is hot. I defy you to give me a better reason.