For decades, travelers have played the "License Plate Game," especially on those longer than usual roadtrips. Sound off! Which ones do you like?
Which ones can't you stand to look at?? 
My top 5 are:
1. Maine (best)
2. Colorado
3. Minnesota
4. Rhode Island
5. Alaska
My bottom 5 are:
1. Pennsylvania (worst)
2. Indiana
3. Michigan
4. Georgia
5. Ohio
The basic Michigan isn't quite so great, IMHO, but the one with the Great Lakes on it is excellent (IIRC, it's a couple of bucks more).
My top 5 (no order):
Arizona (nice desert scene)
Florida (what California should aspire to)
Hawai'i (simple, elegant, pretty, unique)
Iowa (easy to read, nice background)
Utah (Delicate Arch is always a nice touch)
My bottom 5 (again, no order):
California (can it be more bland?) (maybe the worst plate out there, IMHO)
Delaware (again, bland, but at least it has color)
Massachusetts (almost as bad as California)
Pennsylvania (could be worse - see California)
West Virginia (again, bland, but could be worse - see California)