Frontier - http://papa.org/play/games/?pin=frontier&tutorial=false&gameplay=true&competitive=false
Skateball - http://papa.org/play/games/?pin=skateball&tutorial=false&gameplay=true&competitive=true
Both games have very similar rulesets with perfect risk/reward attributes. You've gotta build bonus/bonusX then qualify and collect bonus. The games never feel like they top out, because once you've maxed bonus, you still have to qualify and re-collect, which is dangerous on both machines. The shots and rules on both of these are perfect.