1 - Playboy - 'muff said. Classic artwork and a game that holds its value. When Hugh has his last hug from his current wife, then this machine will double in value.
2 - Eight Ball - Absolute iconic pinball machine. Chimes, billiard theme and the Fonz on the backglass.
3 - Xenon - In terms of value, this could be #2, but I put it a #3 because I like the Fonz. I think this Bally is often over rated, and then I just stare at the artwork..
4 - Star Trek - picking up in value. A better player than #1, #2 & #3, but not as valuable.
5 - Flash Gordon - The best player of all the games. Often trashed playfields but perfect backglasses.

If I owned all five games, I'd throw the $ at the Playboy as a keeper, but Flash Gordon as the best player. I'd play all games equally, but would gravitate towards the Flash Gordon if I wanted a "serious" game. Xenon is a really nice artwork and themed machine. Actually, they all seem to sit well with each other for different reasons. I think there's a good mix of diversity, and I would not be at all surprised if this was your aim ?