When it comes to headphones, I rely on the ultimate headphone geeks, the guys at Headroom. They have a page comparing all the headphones they sell. If you just want inexpensive sealed 'phones, they love the Sennheiser HD201 ($20).  

If you want real serious isolation from surrounding noise, the Shure E2c is the place to start (available for around $65 on sale at Buy.com). These are great sounding, and much more effective at blocking ambient noise than typical "noise cancelling" headphones. But you don't want to try riding a bike with them -- too much isolation from your surroundings. For a bit more money, I think the Etymotic ER-6i ($120) is more comfortable than the Shure's. I've owned both, and ultimately opted for the Etymotics for comfort reasons. YMMV.

As an audio engineer, I'm never without my Sony MDR-7506's, and I think they can be found for about $90. They are very accurate, and sound amazing. For casual listening I preferred the older MDR-V6's because they are a little warmer (not, however, the V600 replacement = too muddy). And while the V6 is better for listening as a fan, I still choose the 7506 for critical listening. A bunch of guys in my jazz band have been snapping up the new Sennheiser noise canceling headphones ($160?), and apparently they compare favorably against the Bose noise canceling for about half the price. Great for air travel and bus rides...

I have a pair of Sony's MDR-V500 headphones, and I love them. They also have the big padded ears, and I can find them at any half-decent electronics store around here. They're about $100 CDN (~$80 US).

I'm looking for some headphones that fulfill two criteria above all else-- blocking out external white noise, and being comfortable to wear. The actual quality of sound reproduction matters to me, but not nearly as much as you'd think.  

I tried a friend's Sony MDR-CD180s, with enormous pads around each ear, and they were great. He also assured me they were cheap. However, they seem to no longer be made. Can anyone recommend something along the same lines?