I like the Blob. I think it's a better monster than zombies, locust swarms, or anything else that just devours/replicates it's way through everything.
The Thing comes in a close second. Infecting, replacing, then merging or splitting as needed to survive and repeat.
The Corpus from Mighty Max that absorbed people and got bigger and bigger.
Also Hive minds can get fairly disturbing. I'm not talking about the Borg. I actually liked the borg. What I don't like about the Hive mind monster is the feeling that everyone is in on it but you.
I just realized that my fav monsters are the ones who absorb you into the whole whether it's physically or mentally. I must ponder this further.