I'd have to go with either "Shoop" or "Sweet Child of Mine"
Also, "Justify My Thug"
I'd say "Encore" by Jay-Z.
"What the hell are you waiting for? After me there shall be no more..." Just general "I'm #1!!" song.
COLD TURKEY, by John Lennon. You do have to know the song to Know what I mean, but thoroughly and real life-ingly horrifying.
"Monkey Wrench", by Foo Fighters. One of the best opening riffs ever. Good for exam psych-ups. 
And I know that either Tino or some other Yankee uses it, but I am hereby taking it back: "Baba O'Reilly", by the Who. I'd coopt Pete Townshend's guitar solo in the middle.
I guess those would both be AB music, though. Hmmmmm......striking terror....
The Overture to Don Giovanni is pretty damn terrifying, so I'd probably go with that.
This is way, way out of date, but my baseball fantasy used to include Queen Latifah's "Ladies First" coming over the loudspeakers as I threw my warmup pitches (or sauntered into the batter's box, depending on whether I was playing shortstop or coming in as the closing pitcher in that particular edition of the baseball fantasy).
And now, I will have that song stuck in my head for the entire walk home. "A woman can bear you, break you, take you..."
Gimme Shelter snaps first to mind, but to really put a bend in the mind of any upcoming batter or hitter, I gotta go with Mofo by U2. Play it loud.
something by metallica...probably sad but true or master of puppets.
or something by NIN...march of the pigs or somewhat damaged. big loud angry snarly songs.
Something completely bizarre and inexplicable, like a Jamiroquai song or Franz Ferdinand. Something that would make everyone in the park just sort of sit back for a second and say to themselves, "Wait, what the fuck?"
I am so old, I don't even know most of these musical references, but hey, if you want to mess with the batter's head, why not Mandy by Barry Manilow? Imagine trying to concentrate on a pitch with that crap ringing in your ears!
If I was at the plate and basegirl was on the mound, I'd definitely want American Woman by The Guess Who blaring over the loudspeakers.
a Jamiroquai song or Franz Ferdinand.
Thanks for making me snigger audibly in the office 
Like Steve, I don't recognize most of the musical references here, and yes, it's partly because I'm old, but mostly because I haven't listened to the radio in a very long time.
If I were a closing pitcher: Wagner's "Flight of the Valkyries."
As a slugger: AC/DC's "Hell's Bells."
Wait, wait, wait: Hell's Bells is already taken. Shit.
Okay: Hole's "Awful."
"Money" -pink floyd
with extra bonus points for showing the synced Wizard of Oz clip on the jumbotron
As a slugger in a 'trying to recover from a deficit' situation: "I'm only happy when it rains"...The bravado of the line "pour your misery down on me" (though probably not the meaning the artist intended for it) should give the hurler cause to be nervous...As a hurler: "Rubber Ducky" I think it was done by Jim Henson of muppets fame, very hazy on that one. It would be very distracting to a hitter to think that the pitcher is fond of him (her) like a bath toy...
I wanted to play baseball until I was nine, not having, apparently, recognized that I was a girl. Anyway, I'm big into wuss rock and not into much else, so I'd have to pick some Celtic-rock monstrosity that everyone else would probably hate. I'm a dork like that.
Andraste, the Wagner is an awesome idea. I see Robert Duvall in aviator glasses! I love the smell of napalm in the morning!
TGIC and Andraste: I had the same mental image at the Wagner idea. It'd certainly be an intimidating enough subliminal thought for anyone who's seen Apocalypse Now.
For me, I would pick "O Fortuna" from Carmina Burana as my entrance song as a closing pitcher. Good pump up tune, and some mildly-ironic lyrics (once you translate the latin).
Hot Stuff - Donna Summer
If I was a closer: "Talk Dirty to Me," by Poison.
If I was a slugger: "Glory Days." Specifically the opening.
The best is still Brandon Inge... I forget the song, because I suck at rap, but it had the line "wish I was a little bit taller" in it, and that was his AB music, and I died a little bit laughing every time he came up.
Fiyo on the Bayou by the Nevilles
(cause I'm reppin NOLA, ya dig?
As a summer resident of San Diego, "Hell's Bells" was just such a nasty closing song (not to mention one of his warm-up pitches is a running blast from second base...crazy). He'd save the game, fireworks would light up the sky, and I left Qualcomm with the satisfaction of only paying $5 for the experience. And with that, my personal music would be Steve Brady's rendition of "When She Comes".
ummmm - never mind this is definitely out of my league.
Black Betty !!!! would be the one I can think of
How about "Break on Through" by the Doors? I forgot about that.
ooh, the doors definitely. if for nothing than the image of jim morrison half-naked, shrieking into the microphone during one of his (many) drunken concerts. oh, how i loved him in high school (even though he died 6 years before i was even born).
i love the franz ferdinand idea though. that would be hilarious.
I was going to suggest "When the music's over" by The Doors, (turn out the light, turn out the light)for a closing pitcher, but didn't. Now, Kristen, you yourself suggest a song from one of my favorite bands...I love you more with each post I see. Just the refrain of 'Light My Fire' would be good for a DH like Ortiz.
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