Fruit Machine Mania (PS2): All 6 slots, 'reel holds' and bonus games.
If these slots were part of a general casino title, they would add something unique. Particularly with the 'reel holds' (rare in US slot machines) and the bonus games. However once you unlock all the machines in this game, there's really nothing else to do. I guess you could try for a high score however that will depend on luck rather than your skill level.
The best way to unlock all the machines is to stick with the 'Card Master' machine throughout. Set the Risk between 1-3 and rack up the additional Turns in the bonus game. You will gain small scores along the way that will add up. If the machine stops awarding scores, drop the risk to '0' and just spin for free until you hit a winning combination or the bonus game (you won't win anything with 0 Risk but you will preserve your Turns). This method takes around 25-30 minutes. You will also need luck on your side!
0:01 Card Master
1:11 Sea Treasure
2:17 World Travel
3:26 Casino Fortune
4:05 Winner
5:12 Super Slot