I used to think the Predator was the hands down winner out of these - you're almost guaranteed to get at least one kill, it's fast and will get you up to a higher killstreak in seconds, without you having to wait for care packages. I also found that Sentries got destroyed before they got a kill.
But I decided I wanted to get all the Sentry Gun Titles and Emblems, so I equipped it, and now that I know the good places to put them, I find they're a match for, if not better than Predators. In Objective-based games, I can reliably get 3-4 kills for every sentry gun, with the added bonus of keeping enemies off the objective, and forcing them to move in different directions. If you deploy one overlooking a bomb in Demolition, it should rack up 5 or 6 before getting destroyed or expiring. The only setback is that they don't help you get Chopper gunners etc, and if you're unlucky someone with cold-blooded will destroy it before it gets any kills.
So what's your preference?