What I'd like to see is triangle or torx take off as being standard. While square is pretty good, and it's a Canadian invention, one issue it does have is that there are way too many different sizes. Open a bit set and there are like 30 different sizes that are only slightly different. If you don't put in the EXACT right one, it will strip fairly easily. I guess one easy way to solve it would be to have more defined standards for the size of the actual hole so that there is less chance of putting wrong size bit in it.
Flat is ok for stuff that you have to put in by hand, they are less likely to strip than star. The issue with flat is that it's easy for the screw driver to slip out, and don't even bother trying to use a drill. What I do like is "electrical" screws like for inside electrical panels, wiring boxes, etc. They tend to be compatible with flat, square and star because of the design.