I have many cans because I like to have at least 1 headphone from each manufacturer; this way I get to sample each brand's unique sound. I try to get the best can for the money, kind of a lesser version of my dad's system. Currently I have:
AKG K-501 (most comfortable can I own, great for classical and big band)
AT ATH-W100 (good for acoustic jazz, looks impressive)
Beyer DT-880 (amazing with female vocals, comfortable)
Ety ER-4P (nothing else like it sound wise, isolates very well)
Grado SR-225 (the #1 rock can, IMHO)
Koss PortaPro (cheap and good portable)
Philips HP170 ($10 and kicks ass)
Sennheiser HD-580 (best overall can I own)
Sony MDR-V6 (very comfortable with Beyer pads and decently isolative)
All these cans offer different glimpses into my music and each has its strengths and weaknesses. If I had to live with only one I would go with the HD-580. If I had unlimited funds my cans would be:
AKG K1000 (I love this can, very unique experience)
AT ATH-W2002 (much better than the W100)
Beyer DT-880 (see above)
Ety ER-4P/S (see above)
Grado HP-1 (most resolving can I've ever heard; it's my favorite can sound wise, the comfort factor is another issue)
Sennheiser HD-650 (the HD-580 that I love + improvements)
Sony MDR-R10 ('nuff said, though I’ve never heard it)
Stax Omega II (I love this headphone; it's my 2nd favorite can)