I have played SSK1, 2 and 3. Eventually I'll play Ultimate 11 on MVS, but I can't comment on that game, although some people seem to dig it a lot.
For me personally, I see SSK3 as the best game of the series. While it is true that SSK3 doesn't add a lot of new features in comparison to SSK2, I like the presentation best. Some special moves and post-goal victory screens are downright hilarious. The music is pretty good, and gameplay is fluid. It has a learning curve though, especially against the CPU. Against a human opponent it represents some of the best times I've ever had playing video games. You'll find yourself trash-talking, humiliating your opponent with outrageous foul tackles, and screaming out loud as you try to score a goal from a few meters away but the goalkeeper just won't stop making all these great saves.
So, in short, highly recommend SSK3.