Sennheiser CX-300, far and away. Those Senns are probably the best you'll get under $50, however another contender would be the JVC Marshmellow which sounded better than my $120 Shure earbuds after a little tweak (can find instructions for that on head-fi, if you're interested).
CX300 sounds like the better pair here. you could also consider Creative's EP630 or 830. The EP630 is supposedly similar in sound to the CX300. The J2 might be #3 in this group, user reviews even make the first Jbuds out to be better than them! All in all, the CX300 is cheap and sounds great