"I'll be back." is a famous line from the Terminator franchise. It is said in some form in all of the films.
The American Film Institute lists it as the #37 top movie quote of all time in its "100 Years...100 Movie Quotes". 
- The T-800 went into a Los Angeles police station to look for Sarah Connor. He went to the receptionist and said he was a "friend" of hers. The policeman behind the window informed him he'd have to wait. In response, the Terminator says "I'll be back." He later drives its car through the police station and starts a shoot-out.
Terminator 2: Judgement Day
- Whether intentional or not, Tim, John's friend, says "I'm going to get some quarters, I'll be back, alright?".
- After Sarah Connor. John Connor and the T-800 blew up the Cyberdyne Building, the three went down an elevator. The SWAT team shot smoke grenades at them. They only had one gas mask and John and Sarah took turns with it while tear gas filled the elevator. The T-800 said to them, "Stay here, I'll be back." It went up to the SWAT team with only a pistol. It injured them all with it and a smoke grenade launcher to the point where they were incapacitated.
- When Kate asks John what she tell his troops when they realize he has left to infiltrate Skynet to rescue the prisoners, John answers, "I'll be back". 
- Tim is the first human character who says the line (although possibly unintentionally), and John Connor is the second.
- "I'll be back" has never actually been said in Terminator 3: Rise of the Machines, although two very similar phrases have been said.
- When the T-850, John Connor, and Kate Brewster went into the CRS complex and met up with the T-X, the T-850 blasted it into a large vent and then stated "She'll be back". In the same film Kate Brewster says "I'll Be Back by the time you get up." to her fiance Mason before leaving their house in order to get to the Vet.
- When the T-850 crashed into the Crystal Peak base with a large helicopter, it got out, extremely damaged and said "I'm back."
- "I'm back." is also said by John Connor in Terminator: Infinity.