Oh Lord, I can't choose. There has been at least one game in each that I absolutely loved... Each offers a different experience.
I love Splinter Cell for its "sneaky-sneaky-gonna-lop-your-fuckin-head-off" attitude. I love GRAW for its FPS+squad playablility (and its story, which is fun) and R6 for its tactical nature and good multiplayer.
If I had to choose one for next gen, it would be GRAW. The SC for 360 was decent but mostly "meh". Conviction is back with the team that created the good SC games (1 & 3) so it should return to greatness. R6 was good but the multiplayer didn't hook me. Gears offered more that catered to my taste at the time. GRAW was the single-player experience that I enjoyed most this generation, though I'm sure there are many that will vehemently disagree with my opinion.