I played through all of Gears 1, 2, 3 and Judgment during the winter holidays. I liked them more than the newer ones (4 and 5). This is now the videogame series I have played the most of so far. Only Gears: Tactics is still in the backlog.
Gears 1 had the nicest variety in boss fights.
Gears 2 was probably the most fun, with the worm battle and so on.
Gears 3 is just a blur in my memory right now but it served as a good conclusion to the series
Judgment had some interesting tweaks and new weapons to keep things fresh.