the fast zombie is the best zombie if you're not a donator/admin. Now that you're an admin, MOST admins/donators prefer half zombie, while some use the stalker which has a lot of hp i think. The witch zombie is supposedly jumps higher and it might have but i couldn't tell that well. The other zombie is an old zombie we use to let players use but now it's admin only (kinda sucks too ).
Notice how it's on ZOMBIE ESCAPE. Erm, honestly, speed isn't always the best, you have to take in all factors. I personally think half zombie is bad, because of the huge knockback and low hp, however, it still very good if you are at a low holding spot, as humans will have to shoot the floor resulting in other zombies getting pushed less. I use host zombie myself, it's rather quick, looks nice and it has nice hp and knockback. Bloodsucker is kind of not good unless theres maps where people constantly get left behind.
Ray told me that zombies have normal speeds, so they all are the same until a source update deals with this issue