Of course that depends on what your priorities are. If you are looking primarily for ease of use, there are at least two new build systems that hook into the filesystem to automatically track dependencies in a language agnostic fashion.
One is tup:
http://gittup.org/tup/
and the other is fabricate:
http://code.google.com/p/fabricate/
The one that seems to be the best performing, portable, and mature (and the one I have actually used) is tup. The guy who wrote it even maintains a toy linux distro where everything is a git submodule, and everything (including the kernel) is build with tup. From what I've read about the kernel's build system, this is quite an accomplishment.
Also, Tup cleans up old targets and other cruft, and can automatically maintain your .gitignore files. The result is that it becomes trivial to experiment with the layout and names of your targets, and you can confidently jump between git revisions without rebuilding everything. It's written in C.
If you know haskell and are looking for something for very advanced use cases, check out shake:
http://community.haskell.org/~ndm/shake/
Update: I haven't tried it, but this new "buildsome" tool also hooks into the filesystem, and was inspired by tup, so is relevant:
https://github.com/ElastiLotem/buildsome
