Being "one of those finicky assholes who spends a lot of time and money on headphones," I've spent a lot of time and money on headphones over the past couple of years. I've collected my fair share of cans, swapped them out and had them stolen from me, and I've finally and definitively selected what I believe to be the best headphones in the "Portable" "Midsize" and "Fullsize" headphone classes; it's a given, of course, that every headphone aficionado should own at least one pair of cans in each class. Here are my three choices, for you.
In the Portable Class:
Apple Earbuds. That's right. I don't care that you don't have an Apple MP3 player, you should still own these earbuds; here's why: for one thing, I don't want to hear your shit about Bose in-ear headphones and their ilk / price-class. Fuck you. You spent too much on them. Right now, you can pick up a pair of Apple's buds for like five bucks on eBay. Let's consider the basics here: in my experience, these things have a life span of about 8 months given just the basic amount of care. I've got a pair now that's sounded fine for over a year, and they've taken a beating. I've spent more than 5 bucks on earbuds that stopped working within a month, and it makes me mad every time I think about it.
It's not just the price and disposability that makes Apple Earbuds pretty awesome though, it's also the sound quality. Yeah, I said it. The sound quality. No, they don't sound "good," but if you want to find buds that match Apple's in terms of sound quality, you're going to be spending $20+ and if you're spending $20+ on pair of portable headphones, smh, you're making your biggest mistake. Portable headphones are good for one thing: short walks with an MP3 player, and, newsflash: MP3 players sound like shit. They don't drive headphones the way they're meant to be driven, and don't deserve or need anything better than Apple earbuds. If you're looking for a pair of nice sounding headphones for something like the library or an airplane, you shouldn't be looking for earbuds anyway. If you care about how your music sounds then don't spend stupid amounts of cash on speakers that are smaller than your pinky, buy the Apple buds for short walks, and invest in some mid-size cans for chilling in public places.
Lastly, there's the whole "style" thing. I know, this isn't important, it's the music that counts, blah blah blah. But it should be noted that stylistically, these things do one thing right, and it's just about the only thing that earbuds SHOULD do right. They stay out of the way. They're small, white, and so ubiquitous that most of the time I don't even notice people wearing them. The same can't be said for those hideously bulky "Skull Candy" buds that probably cost you more and definitely sound worse.
So here's what I'm saying to you: Apple's earbuds are dirt cheap and they're better than most of the earbuds out there that are less than $20. Hell, they're better than most buds in the <$40 range. So do yourself a favor, and make your portable cans Apple earbuds.
In the Midsize Class:
I think the concept of Midsize headphones is lost on a lot of people, which is a shame, because they're key players in my opinion. Often looked down upon as "earbuds for people who care too much," which is, er, sort of the true, Midsize cans actually excel in a lot of areas that that reasonably priced earbuds don't touch, like sound quality for example. Midsizers do, however, come with their share of limitations: for one thing, they're bigger than Portable cans. Is that obvious? Maybe not: they're actually relatively small, which might give your average mogul the impression that they're good for, oh say, short walks with an mp3 player. Which they're not. I have a pair of Koss Porta Pros which run for about thirty bucks, and I can't stand carrying them around with me in my pocket or around my neck. And yet, I never leave them behind for plane-rides, trips to the library, and other occasions which allow for extended "chilling," which, as we all know, is best served with good music. The reason? I don't want to actually listen to music on earbuds, and when portability isn't a factor, I'll opt for something bigger and better. And the Porta Pros are fantastic by the way.
In terms of comfort, it doesn't get a whole lot better- the Sennheiser PX 100s are actually just a tad bit more comfy, but they're thirty dollars more... and feel about five dollars better. In terms of sound quality, the Porta Pros blow the Senns away; don't take my word for it, check them out online, they're unanimously loved for a good reason. I don't think much more needs to be said for Midrange cans. If you're going in for a pair, and you should, go in for the Koss Porta Pros, with their hilariously dorky 80s aesthetic, nice, round bass, shockingly open-sounding L/R soundstage, and general unreasonable value.
In the Fullsize Class:
Here we have the Audio-Technica M50s. I just picked up a pair of these babys and I'm still working on burning them in, but after a good 10 hours of listening, I can already say that they're leaps and bounds better than the Sennheiser HD280s which were my go to cans for mixing and at-home listening... before they were stolen out of my bag on the subway. So there's a great example of why you should have three pairs of cans: if you get stuck with a single, expensive pair of headphones, you just might try and bring them along, and then they just might get stolen. Or broken. And then you just might cry yourself to sleep (not that I cried myself to sleep after I lost my 280s or anything.)
Good fullsize cans are useful for two things: studio work, and listening to music at home. Generally, they're going to be responsive enough that you'll be able to distinguish between high and low bitrate music, so they're no good mated with an MP3 player, unless you keep a small collection of high bitrate music on your mp3 player... you weirdo. Another factor that has to be taken into consideration when looking for Fullsize cans is amplification- some headphones, like the above-mentioned M50s and Grado's selection sound fine with on-board amplification from iPods and computers, but some cans with higher impedance require an external headphone amp, a nice soundcard or a good receiver to really shine. Bored yet? My point is, Fullsize cans are best left at home, where they sound the best, stay out of your way, and probably wont get messed up or stolen.
I've tested the M50s against my old Sennheiser HD280s, a pair of Senn HD595s, and AKG K-240s, and the M50s are clear winners- they've got a cleaner bass and treble response, smoother mids, they're more comfortable, and their frequency response is just flat-out flatter. I've yet to find a pair of Fullsize cans I like better, but then, I work in the >$200 price range... still, I can't imagine a pair of phones sounding $100 better. Feel free to let me know if you're interested in more A/B tests.
If you're here on this site, reading this, you really ought to have three pairs of headphones. My choices? The Apple Earbud, the Koss Porta Pro, and the ATH-M50. I strongly recommend the first two, but remember, there are TONS of great fullsize cans out there, I just prefer the M50s. Regardless of which headphones suit you, make sure that you've got a solid lineup that'll keep you groovin' regardless of where you are and what you're doin'.