The most important decision that you'll ever make.....

T-800 Terminator, no ifs, ands, or buts.

Why?

The single-mindedness every version of it has displayed in the franchise's history to carry out it's mission, combined with the ruggedness with which they are built gives it the largest probability of success, especially given how calculating it is and how quickly it learns.

Venom would be #2, simply because he's the most powerful and most able to blend in. If I wasn't confident in the T-800's abilities when they are forced to go head-to-head, he would have probably been my pick, and is the only one I'm actually worried about if I have T-800 as a protector.

After that it gets murky. Predator blends in incredibly well, but he's only about as strong as a really fit human give-or-take, and relies exclusively on weaponry that can be forced from his hands. Robocop would've been an obvious choice if it wasn't for how slowly he moves and the exposed organic elements that should make an excellent marksman like Predator or the T-800 able to kill him in a single shot with a variety of weapons. Alien's speed and power are probably the best of the group, but they act mostly on instinct and are relatively easy to fool. That just leaves the being in the middle of the bottom row, and since I have no idea what it is or what it's capable of? That leaves me with the T-800 as my choice for protector.