Would you like a list of all 100+ distrobution's that fit that description? Really the first 5 hits on google would give you some options. Besides, there is no 'best', everything is subjective.
Anyway here is a list of some of the popular ones. http://en.wikipedia.org/wiki/Lightweight_Linux_distribution
I suggest you try to:
1) decide on a package manager that you like (dpkg, pacman, rpm,..)
2) decide on what features the distro comes with (kernel version(s), systemd, ..)
2) decide on a window manager/enviorment you like (openbox, xfce, lxde, awesome, compiz, fluxbox, icewm..)
3) then finally find a distrobution that rolls all the above