Oh, man... There's SO many! Uhm... Everybody who's been reading along so far probably knows my preferences as far a Transformers, so I'm going to pick from the non-Transformer pool this time. Now you already mentioned Terminator, which is truthfully one of my favorite robots of all time... but since I like to try to be original, I'll go with Jack from the Tekken series. He doesn't even have that much to offer in the way of humor or personality. He just looks really cool and can kick some serious butt!
I'd credit him with somewhere between 1/4 to 1/3 of my total lifetime wins in Tekken. There's a small handful of characters that my buddies worry about me picking... but Jack is the only character they will all admit they truly fear and loathe. One time, it looked like I was going to completely dominate and go undefeated all night long. The only thing that broke my streak was when my buddy Jon picked the old wrinkly guy, Heihachi, and chose his alternate costume. (It was a sumo diaper.) Needless to say, I was plenty offended... and such distraction ultimately led to a tug-of-war victory for Jon. After three rounds of frantic button-mashing and hysteric fits of laughter, Jon and the other guys raised their fists high and breathed a huge sigh of relief... then promptly switched the game... to one I've always sucked at!
Jack's greatness is not only based on my opinion. Some Tekkens feature multiple different "Jack" models because his move list is such a cornersone in the series. Tekken four actually took Jack out and I boycotted it entirely! I wouldn't give it one red cent of my money. I played it over Jon's house and it felt not only empty, but vitally broken. It seemed like whoever could tap the buttons the fastest was automatically the winner, because everyone's stats were quick and weak. They brought in some new guy to be the slow, powerhouse type, but he was no substitute for the Jack pedigree. Tekken 6 has a new Jack, so I'm holding my breath... but I'm also wincing... because supposedly Tekken 6 will have another robot called R.O.S.I.E. who's supposed to be the "Jack beater." At first, I'd say it's a stupid idea, but then again... how important of a robot do you have to be in your franchise to warrant your very own gargantuan nemesis? Half the game's basically built around the guy... er... bot.