Dredd's underbite is scarier than all those other things combined.
How about Transformers vs. Terminators/Skynet? Transformers vs. the Predators? The preds have the tech to come up with strong enough weps and gear to hunt them. Or better yet, Transformers vs. Aliens. The Aliens have the acid blood and numbers to cause a problem, also if its set on earth theres plenty of breeding stock for the xenos. Just my 2 cents on what i think could be epic stories if done correctly.
We are complete agreeal. I want to see xenomorphs go against TFs, that'd be awesome.
Better yet, BEAST WARS VS ALIENS!
The Beast Wars bots are part organic (I guess) so imagine a poor Maximal character is found hurt and is brought back into the base. When Primal and crew try to nurse him and interrogate him, BAM! CHESTBURSTER! A few days later, the Maximals are being killed and eaten one by one by something more horrible and more nightmarish than any Predacons ever.
Something from hell that transform.................
Of course, what I want is something that emulate the very first film. The Predacons should be out of the picture. Only ONE monster is needed. We must taste the FEAR of these Transformers. Finally, when the creature reveal itself at the end, it must have 3 or 4 modes, each ones more nightmarish than the others (à la RID Megatron). Everything will culminate as a final battle between our hero (no nessesarely the Maximal leader) and the Alien.