On the one hand, such a question can be answered as:
Which headphone(s) gave the best sound quality FOR THE MONEY?
This form of the question would be answered by me with:
KSC75, AKG K240S, AKG K501
Each of these phones gives many aspects of sound quality only available in phones from other manufacturers for multiples of their prices.
HOWEVER, I only use KSC75 for music which I only have access to at low bitrates or on low quality recordings - so my USE of it is not very high or often. I used each of the AKG K240S and AKG K501 as my main can for about six months, and then tapered off until I basically gave each away, each about a year after originally buying them. Whenever I'd listen to each of these AKG phones, I ALWAYS came away saying, "MAN, these phones sound really GOOD (for what I paid)." But, when it came to actually listening to my music, and wanting to experience that music in a way most meaningful to me, I soon found myself reaching for another phone. For folks who find the sound of these AKG phones to deliver, long-term, that meaningful sound that they reach for, then, for their sonic qualities and price, they can't be beat. But, for me, for what I paid for them for six months or so of use, how good WAS the bang for buck?
On the other hand, if best bang for buck has to do with how much USE I get from a phone for what I paid for it, I've got to say that, so far, my Grado SR225 have gotten three years of solid use (with MUCH music really enjoyed through them), and the HD580 I bought about a year ago are now solidly ensconced on the long-term listening tool (easily reachable) part of my listening desk area. Since I fitted these with HD600 grills and a Cardas replacement cable, they are not low-cost phones at this point. However, for the amount (frequency and time span) of USE (with high enjoyment) I'll get from them for what I paid, I have to say that I'm projecting them to deliver VERY good bang for buck.
I've said this before, and I'll repeat it here. I've read many posts reporting the HD580 and SR225 as being sonic polar opposites. I find them to be VERY complementary because of:
a) What they have in COMMON. In switching between them, I don't lose very much of what I love in the one to get what I love about the other.
b) What they don't have in common - when I switch from one to the other, I get strengths in each that are not present in the other - if not, why switch?
So, having the two extends the range of music I can listen to with optimal enjoyment. In short, I find them to be highly complementary.
But polar opposites? Not in my rig, to my ear...
Taking a step back:
There are some great recos in this thread. For everyone looking for their next step in phonage, enjoy the embarrassment of riches being pointed to.
