Which X server?

So I'm familiar with the fact that many window mangers exist but wasn't
aware of the prevalence of different X servers. :-
http://en.wikipedia.org/wiki/Category:X_servers

Being as my current linux mission is efficiency on old hardware which X
server will best suit? I understand puppy linux uses Xvesa but understand
this is fast but will run all code without discrimination and so potentially
unsafe. Comments regarding this please.

I presume XFree86 is the most well developed?
Do any other servers compare in terms of features?


Thanks


.