There are lots of great gun games, and they were made by so many manufacturers for so many years that there are a lot of models out there. As mentioned, check out pinrepair to see the wide variety.
I have a Twin Rifle and agree that it's a great game - very fast and competitive, but you can also play a single player game on it as well. Another two rifle game that hasn't been mentioned yet is Midway's Twin Pirate - it uses a different target system (light sensitive targets) than the typical dale gun games which are more common.
There are some great single player games out there. Both CC Shoot Out and Coney Island are great games - they both have a variety of target styles (pop-up targets, light targets, holographic targets) which makes for a great mix. Some of the earlier games were much simpler, but can still be fun.
Personally, I like gun games that work on a timer rather than a number of shots per game - play is faster and, to me, more exciting. Games with 8 track sounds are fun too - it adds to the gameplay.
Projectile gun games, like Bally Sharpshooter and Midway Shooting gallery, are very different since they shoot pellets at the targets. They are big, single player games, but are very cool.
Some things to compare on gun games:
1) Gun type: dale gun vs. light gun vs. projectile
2) Game play: clock timed vs. number of shots
3) Sounds: Simple gun firing vs. background music & sounds
4) Gun recoil vs. no recoil
5) Themes: Lots of variety
6) Targets: stationery vs. moving
7) Target Style: Pop up, Holographics, Light targets or some combination
8) Game type: single player vs. two player