There's a very detailed discussion of professional grade headphone selection and use in the "Production Tips" sticky of the "2488 and DP-24/32" forum. You can find the specific post in the index to that sticky.
If after reading the Production Tips post, you have question(s) about a specific headphone you have an interest in (either those mentioned in the sticky post, or any other), come back to this thread and I'll address it if I can.
But as this is a general topic not specific to the portastudios, you may want to ask the moderators to move it to the Recording 101 forum so you'll get a broader range of exposure and feedback.
FYI, I second David's recommendation of the 7506 for tracking.