I was going to say Sennheiser HD5__, AT M50x, or Grado SR80e, so it looks like everyone's pretty well covered it. That said, I've listened to all three and might be able to say a bit more about them.
The Sennheisers are a pretty good default, IMHO. They don't impress quite like the others, but they also have a notable lack of weaknesses. Not impressive mainly means they aren't as fast as the others and just can't resolve quite as much detail. The shape of the FR band is also a factor - they're definitely the warm and bassy kind of headphone, not the analytical kind. It's a tight and well-controlled kind of bass though. They do leak some sound, since you said that was an issue, but not as much as Grados.
The Audio-Technicas are the right choice as far as isolation (including not leaking sound). They're somewhere between the other two in FR shape, and not too far behind the Grados in speed/detail. They do have a sound quality flaw, but it's a subtle one that's tough to describe. My guess is that it's something to do with phasing and/or particularly variable transient response at different frequencies. I would describe the result as slightly brittle or edgy, but it isn't confined to high frequencies like those terms imply. The FR band is excellent, and they don't seem at all resonant or harsh.
Grado SR80es are what I use. They leak a lot of sound, have a more analytical FR shape, are crazy fast/detailed, still sound smooth in the top end, and are cheaper than the others. They have one big weakness: a giant resonant peak at 2 kHz. Fortunately, that resonance seems to be between the back of the driver and the grills to the outside, and removing the grills fixes it right up. Of course, you can then add "they look weird" to the list of disadvantages. Obviously they're not for everyone, but I wouldn't trade mine for anything less than HD600-class. I hear that Grados don't always get purely better as you go higher-end - if I had $150 instead of my current pair, I'd be looking to buy another pair of SR80es, not SR125es.
Any of these are good enough to benefit from a decent source, but Grados should do so more than the ATs which should do so more than the Sennheisers. If you know you'll be using a mobo's audio or similar, that's a point in favor of Sennheiser and against Grado.