Etymotic Research
Basically anything from these guys is far and away the best in terms of isolation.
I'd highly recommend going used (most tips are silicon so can be washed thoroughly, theyre also all replaceable)..
For example;
HF5 (you'd have to buy your own tips in this case)
The holy grail here would be the ER4. Great isolation, fantastic sound. Weak bass impact, however.
I use the ER4 and find the bass fine, and it offers 33-41dB Isolation. So yeah.
Alternatively, you could look at a Sure SE115
SE115k
Just a quick warning, you might experience hiss with some of the Etymotic research headphones (that is, your mp3 player might produce a high pitched whining noise), due to their sensitivity etc.
The sansa clip doesn't hiss audibly in my experience (or at least no where near what some others do). Not sure how they did it so cheap, maybe an impedance tweak, not sure. You can get sansa clips for like $18
You could also look at Ultimate Ears (some drummers use them, but they're inferior to etymotics IMO). Since they were bought out by logitech theres been a few sales.
herp