It all depends on what kinds of quilt patterns you make most often. I originally thought I should get the odd shapes that would be hard to rotary cut (circles, hearts, rag die) but now I realize that I really need the standard sizes more! I don't do much appliqué and I like to use my scraps too. So here's what I use the most for what I like to sew:
the 5" square, the 2" square, the 2.5" strip cutter (really nice!) and the 3"x6" brick block. If my scraps aren't kept in usable sizes they get out of control very easily!
Here's the drawer next to my machine filled with 2" squares (to use as leaders/enders) that are all cut perfectly and ready to go:
[ATTACH=CONFIG]421146[/ATTACH] and what they become, just because they're handy![ATTACH=CONFIG]421147[/ATTACH]