Man, this is a tough one!
I love the fast paced action, the pressure, the mindgames (damn!), punishment mode (even punishing a single blocked standing hk is awesome) and how you can still see new shit (mostly counters) after all these years
I still have tons and tons of fun when playing good opponents and I hope (no matter how much time has passed) I can still play with someone for many years to come. The day I'm forced to only vs-cpu playing, I'll drop UMK3.
Thank Raiden for our beloved run button!
And now, answering the thread question... my favorite thing about UMK3 is
INVIS!! fuck yeah this is the most badass/awesome/original/fun/ frustration and fear inducing special move in any FG ever! I know, MK2 had invis but in umk3 you have 2 (technically 3 lol) invisible characters and the mindgames are far superior when invis in UMK3. They fucking ruined invis in MK9 :/ can you imagine being invisible MK9 Smoke (real invis) and having all those overheads? lol insane mindgames!