Looking at the alien and zombie shooter games they all look like pretty much the same game just with slightly different iterations. So I was wondering which one is the "best" version. In both cases the 2nd game seems to be the more refined and to have more features.

The Alien Shooter series seems to be the best received and older one. With obviously the 2nd one touting more graphics guns and big booms. So from people who have looked at them or played them, which game is the best?

For me they are the same. Fun to play for a few minutes and than boring.
In a nutshell its you and hordes of enemies walking to you....i mean real hordes. No tactic at all just shooting.
If you don't call the stupid KI a tactical element, like they can't go through a doorway or can't find the open way to
you just 2 m away. Your only get one problem...enough money to buy bigger weapons or ammo (good that there are
easy cheats for this).
And there are bugs and enormous steps in difficult. Sometimes your mission is completed only the program doesn't
let you into the next level. Or you are stuck behind some objects forever (hope you have saved).
And than the big difference in difficult....enemies that just takes forever to die.
So, if you like a game were you in most levels just stand at one or two spots and than trigger away everything in
front of you these are games you may like.