Royal Guard - not least for the moment of choice where you decide to deliberately sacrifice a ball down the middle to get the extra 500 points you need to get the score replay.
Time Zone - possibly the easiest machine to play for free all day; just get the post up and shoot the centre target when the 'time tunnel' counts down to 25,000
Jack in the Box - favourite game I ever owned; I bought it with four or five others from Deith Leisure in 1980 (they were selling hundreds of games off for £20 each when no-one wanted EM's)