It depends on how you want the game to play.

Fast opponents are more suited to "twitch" type games. It is also not an attribute often associated with zombies. Exceptions, of course, exist. Generally, zombies are depicted as a shambling horde. Individually, they are not terribly impressive. But the sheer quantity with virtually unlimited reserves (every casualty increases their numbers) makes them formidable.

I would recommend leaving speed for other opponent types, like giant mutant mosquitos or something :-)

eh, if youre gona make em smart u might as well not even make em zombies

Lots and lots of slow zombies, so they can swarm and surround players. Though maybe every once in awhile you could introduce like one or two fast ones for a tough challenge (dont make them weak, in this case).

I'd say 3. That way, players would have to adapt their battle strategy to their opponent(s) more often. Like have some zombies slow and some fast and some in between and etc...

There are several games with zombies that have varying speeds. One that I can name off of the top of my head is Half Life 2.
Each of HL2's zombies demanded a different combat strategy because of their varying behaviors and speeds. Normal zombies were easily killed because of their lack of speed, Fast zombies would ambush you and attack you from buildings, Poison Zombies were super slow but they made up for it in how they attacked and how much damage they coukld take.

'til you run outta ammo!
You should put limited ammo in the game
Adds to the hardness

I mean, a slow zombie is easy to kill and avoid
But the fast, limber ones?
I'd like a shotgun with ammo. BUT, That is just me.

And yes, a mixture would be good. Make the fast ones have little health, though. Except for a few "Super" zombies. Strength and health of a normal zombie, speed of a fast zombie.

yeaaaaa, i remember running out of ammo in RE2 and getting surrounded by like 5 zombies and trying to take them out with my knife... lets just say it didnt turn out so well!

Well, zombies are "born" from living humans... And humans come in all shapes, sizes, physical conditions, abilities, etc...

So, doesn't it stand to reason that a zombified human would be limited (or enhanced) by the source material? Does being a zombie suddenly allow the body to do things it normally couldn't (well, aside from the whole "living dead" and "impervious to pain" things)?

For instance, a zombie of a flabby couch potato might not move as fast as a zombie of a track sprinter...

And then you can add in effects like missing limbs... A zombie hopping around on one leg, or dragging themselves along because they have no legs isn't going to move as fast as one with two "healthy" legs...

And how about zombies of military personnel or police in body armor, riot gear, etc? Give them more resistance to damage?

Though I'm not sure how detailed you plan to make the zombies in your game (judging from your other discussions about it, I'm guessing this level will be "very"), perhaps all of the above can just be represented by random stats for the zombies...