Oooooh hard to pick! So here I go:
1985: Definitely Gambler. She looked so sexy! Nice and curvy and NOISY.
1987: White Heat. There's something extraordinarily cute about her running around in a tuxedo with baggy pants and a bowler hat. Not to mention the crazy dancing, the funny little gun and the kiss with the dancer where she licks her lips. What a girl.
1989: Express Yourself. But that's for the choreography. Her greatest live performance EVER was at the MTV Awards in 1989. She looked pleasingly chubby, not over-exercised, and she was clearly having the time of her life: watch the look on her face when she does the supremely awesome kicking thing after the running man! (Ahh bless, she doesn't dance like that anymore...)
1993: Hmm. Tough one. I've watched the DVD alot. It's better without sound. And quite nice if you close your eyes.
2001: Ray of Light. For the lights, the sounds and the energy. This performance was surpassed by Ray of Light at Live8. The sound was better, the dancing was hotter and the scream at the end was brilliant.
2004: Bedtime Story! One of my favourite of her music videos, even if it a backdrop and unreleased and has no plot. Who cares, she looks hot in it, and the remix is 'fee-urce'.
2006: Let It Will Be. The DVD made it look eight times more exciting and dynamic than it really was, and it was very well edited, but c'mon! She gyrated, she yelled, she thrashed around and she did her famous high kicks. What more could you ask for?
[Inserts Live Earth's "La Isla Bonita": fantastic remix, funny co-stars, amazing costume and great stage lighting. One of her greatest performances ever]
2008: So far, looks like it's gonna be either Die Another Day or Like a Prayer. The first one for the video, the second for the whole package: killer remix, fantastic screens and great thrashing dancing with the beats.