If you're buying slabbed buy the absolutely highest grade (CGC) with the best page quality that you can find and can afford. If you only want to read them then buy the Marvel Masterworks. Note that books with the same grade are not necessarily all equal. Translated that means buy the book not the CGC grade.
In fact, if you're new to the series, which it appears you may be, you may want to start with the Masterworks, see which issues you like the best and then go after the originals in high grade. Good luck!