How many bands would you consider yourself a hardcore fan of?

For everything else, I'm pretty much content to just own the essential recordings and leave it at that.

PJ Harvey
Helium/Mary timony
Lida Husik
Geraldine Fibbers/Carla Bozulich
Shannon Wright

I have to be completist about at least one band/artist at any given time, or my life is less fulfilling.

Lots of stuff i consider essential, like old REM or Echo and the Bunnymen, I wouldn't say I'm hardcore about. I'm less hardcore currently about PJH but I love her old stuff enough to never get rid of it, and own enough that it still looks sort of obsessive.

U2
Handsome Family
Low
Neko Case
Steely Dan
Yo La Tengo
Stereolab
Underworld

Or Jean Micheal Jarre's "Oxygene".

but i could manage without them.

Those are the artists I'd act the completionist with.

Now that that's out of the way, I can mention the second-placers: Royal Trux and Drive By Truckers.

Past obsessions include Morrissey / Smiths, Whiskeytown, The Fall, and Slayer.

Bob Dylan *
Neil Young
Rolling Stones *
Led Zeppelin *
Miles Davis **
John Coltrane **
Quicksilver Messenger Service *
Prince
George Clinton and projects ***
Fred Anderson
Ornette Coleman **
Anthony Braxton ****
Art Ensemble of Chicago *****
Sun City Girls ******

* I own at least 6 bootlegs each by all these artists.
** yes, I really do have just about everything, thanks to the glory of boxsets. Got both artist's Prestige sets, Miles's complete Columbia and Capitol sets, John's Atlantic and Impulse sets, Miles's Ascenseur Pour L'Echafaud and various 80s sessions, John's Bethlehem recordings and various Blue Note appearances as leader and sideman... Even a few Coltrane boots. Oh, and I'm pretty sure I own close to every note Ornette Coleman has played.
*** got just about everything. All the Parlet albums, all the Horny Horns albums, the Eddie Hazel lp, the Godmomma lp, the George Clinton "Family Recordings" series of CDs ... sheesh, even the "By the Way of the Drum" 12" and the freakin' Jimmy G and the Tackheads lp.
**** I've got everything up until about the mid-80's
***** Ditto (basically, everything up until their stint on DIW)
****** Yeah, I am a collector scum. I really do have just about every note these guys have officially released.

Talk Talk
Orbital
Outkast
Disco Inferno
The Stone Roses
Bark Psychosis

But that's totally subject to change every five minutes, and doesn't take into account all the artists who I love one and a half albums or a few singles or just one damn song by. I really wouldn't want to never be able to hear "Groove Is In The Heart" or 3ft High & Rising or "Lazarus" or Dig Your Own Hole or "I Want You Back" or In A Silent Way again, but there are big chunks of each of those artists' output that I'm not arsed about in the slightest.

Back in the day - the Manic Street Preachers

Also the Boo Radleys, Teenage Fanclub, the Trash Can Sinatras, Jesus and Mary Chain to some fair extent as well.

HAHAHHA...yeaaah...gonna have to think for a bit...

Stooges/Iggy Pop
Replacements/Paul Westerberg
Beat Happening
Clash
Ramones
Stone Roses
The Smiths
Velvet Underground
Cheap Trick
The Clean
X-Japan
Guitar Wolf
Fall
MBV
Pixies
Pavement
GBV
Augustus Pablo/King Tubby
Desmond Dekker
Willie Colón/Hector Lavoe/Rubén Blades
Mano Negra/Manu Chao
Soda Stereo
Los Fabulosos Cadillacs
A Tribe Called Quest
DJ Shadow
Boards of Canada
Husker Du
Devo
The Police (NUT teh Sting!!!)
Men At Work
Misfits
Nirvana

...that's as close to definitive as you're likely to get from me.

T. Rex
Selena
Black Sabbath
Buzzcocks
The Jam
Angelo Badalamenti

Metal:

Countess
Ildjarn
Burzum
Darkthone
Iron Maiden
Katatonia (at least the pre-2000 stuff)

Dance:

M.I.K.E.
Ural 13 Diktators
Legowelt

The Boo Radleys
The Beach Boys
Aphex Twin
Radiohead
Beastie Boys
Plaid
Autechre

Wow, how utterly white of me.

We all know my answer.

Foo Fighters and Killing Joke.

I've been fine without Nirvana. Really. But I've got to listen to Killing Joke on vinyl at least once a day. And I'm a huge Foo Fighter fan and have to have those cds.

I'm more of a 'hardcore' Dave Grohl fan than anything else, really.

Yes
Smashing Pumpkins
Radiohead
Blur

but if we stick to the completist rule, please add:

Throwing Muses
Fugazi
...Trail of Dead
Appliance
Anathema (post-Darren White)
Ben Harper
Pixies
Breeders
Dandy Warhols
Dream Theater
Genesis (Steve Hackett era)
Eels
Foo Fighters
Girls Against Boys
Gomez
Led Zeppelin
Long Fin Killie
Madrugada
Manic Street Preachers
Mogwai
Placebo
Red Hot Chili Peppers
REM
Monster Magnet
Muse
My Dying Bride
Pearl Jam
Porcupine Tree
Poster Children
Red House Painters
Shellac
Shihad
Stone Temple Pilots
Super Furry Animals
Tea Party
Tool
Tori Amos
Type O Negative
Verve
etc....

and I own basically everything bassists Mick Karn or Percy Jones have played on

Morphine
Ween
Outkast
Talking Heads
The Roots

Soloists I'm most fanatical about:

Tom Waits
Cee-Lo
Bjork

Only group I might be willing to kill whilst defending the honor of:

Fishbone

Under control by comparison:
The Damned
Tom Waits

Mere passing adolescent flirtations:
Aztec Camera
Bauhaus
Buzzcocks
Nick Cave
Chumbawamba
Ornette Coleman
The Cure
Miles Davis
The Fall
Flaming Lips
Joy Division
Killing Joke
Magazine
Charles Mingus
Monochrome Set
Penetration
Pere Ubu
Pixies
Prefab Sprout
The Slits
The Smiths
Theatre Of Hate
The Vibrators
Frank Zappa
XTC

Shellac
Fugazi
PJ Harvey
Tool
Nirvana

You know, it's funny, 'cuz I was so apalled by this unrepentant fanboy behavior when I first encountered it my first year of college that I actually dared to investigate further, and thus, the rest is history. As they say!

PITCHBLENDE (own a portion of the original cover art to their last record, take a guess which one!)
BOREDOMS (played first super aRe era material in the united states live at my radio station when i was in the booth)
MY BLOODY VALENTINE (getting my copy of loveless signed by all 4 members)
PAVEMENT (saw them live over 25 times between 1991-1996)
DRIVE LIKE JEHU (own original LPs/7" with custom artwork, saw them live over 12 times)
FURTHER (words can not say)

daft punk, jacques lu cont. maybe just daft punk really though. or maybe even just thomas bangalter.

there are tons of bands I like equally but know that there are WAY more hardcore fans out there:
Pulp
Roxy Music
Spacemen 3
many more.

YOU CALLING ME BORING, BIYATCH?!1

Uhmmm...nice choices, dude...

disco inferno
galaxie 500
kendra smith
the clientele
love as laughter
the fall
the marine girls
the stockholm monsters

and many i forget!

It almost surprises me, that I'm not that hardcore fan of that many bands. Yet, my obssesion for Soundgarden its almost to the point of religious fanatism; you know, ala Alex in NYC with Killing Joke.

elliott smith
wilco
tom waits
opeth
radiohead

got all the above's records and have played them all to death, have seen them live numerous times (except waits :( ), have dug out most of the rarer stuff, etc.

i'd like to include other bands like the cure, my bloody valentine, katatonia, fugazi, but i'm not quite there.

working my way up to:
Beach Boys
Love
Eno

And that's about it, I think.

and 1000 others

There are considerably more Genesis tracks that I dislike (OK, I could have said Genesis 1970-1980 to make it more accurate)

MX-80 (Sound)
Led Zeppelin
Beatles
Love
John Coltrane Quartet (and, eventually, Quintet)
Velvet Underground
P-Funk
Captain Beefheart
minutemen

Black Sabbath, Pere Ubu, Van Halen, Motorhead all coulda made this list but for unfortunate changes in personnel that altered 'em forever. (My support for most bands is mile-wide/inch-thick.)

high school:

The Misfits/Samhain
Throbbing Gristle
Velvet Underground
Swans
Sonic Youth
Einsturzende Neubauten

college:

Nurse With Wound
Enoch Light
Borbetomagus
Autechre
AMM

grad school:

Iannis Xenakis

In terms of sheer number of releases owned it is a tie between Nurse WIth Wound and Enoch Light.