Properly inserting earphones will make a much larger difference in isolation than any change in make or model. Don't just push hard and twist.

How to insert earplugs.

Opening your jaw during the insert/expand phase also helps. A proper seal will do wonders for sound quality and low-end response.

One you insert them properly and block the ear canal, almost all sound to your eardrum is arriving through other paths (you'd be surprised how much your skull transmits sound waves), so a custom fit doesn't do much for isolation (though they are much more comfortable and worth the cost)