Of the models you listed, I prefer the KSC-35 for portable use. They have the best, most balanced sound of all those listed, IMO, they actually have good bass, they are extremely comfortable, and they are great for active use since they don't fall off. Best headphone under $50 to me.
The KSC-50 are very similar to the KSC-35, but they use a new design that is heavier and less comfortable. In addition, they don't fit as snugly, so they tend to fall off easier (and lose a bit of bass). The 2nd-best here, IMO.
The 888s are good, but expensive (IMO) for what you get. Plus, their sound seems to be a love/hate thing.
The Senn MX-500 are great for the price, but suffer from earbud syndrome: lacking in good bass (though more than the 888s), and they tend to fall out if you're too active.
The EX-70 don't sound as good as the 888, 500, or KSCs (even with mods, IMO), but they do offer superior isolation than the others.
The MDR-33 should just be forgotten from the get-go