I have played every RTG game there is and I have found 2 games to be the best value for your money. Loose Caboose has the best bonus round possible. For 25 cents, I have hit bonuses of over $100 several times.
The trick to this game is to learn to stop the last reel. The reel is relatively a short one and you can time the last caboose needed for your bonus quite easily. If you are looking to protect your money because of a bonus playthrough requirement, then Wok and Roll is the best game. There are 2 bonuses instead of just 1.
The bonuses are not that big, but are very consistant to hit. This game will not drain your money like all the rest can and will.