Depends on the specific guitar, but in general, I prefer plain over flame. I can't stand uniform flame - no offense to the (previous) owner, but this one posted in this thread is the worst to me:
But some of the wilder flame posted here is gorgeous! It also depends on the finish - flame with cherry sunburst doesn't do much for me because it's SO overdone, but flame on rootbeer, wine red, natural, amber, and others can look amazing.
Here be my own honeyburst plaintop:
The color is a bit off in this shot, but it shows the grain nicely: