Basicly, what old games are you just as happy to play as your newer ones?
(Not including FF if possible ). They can be part of a series or stand-alone titles.

For me it would have to be Pokemon Blue. i have at least one game from each generation from Heart Gold backwards and despite this i still like playing Blue on my old GBA. To me its the simplicity that this game has. The're 150 pokemon to obtain, as opposed to 500+ of the newer ones, there are only 3 ways of evolving them: leveling up, using a stone and trading, as making them like you first and held items came later. Also i guess its because the games have always stuck to the basic formula, they didnt try to use some new battle system or different way of leveling up (can you guess the reference )

Although there are a few kinks. Your bag, for instance can only hold around 20 items and only has one pocket. To use an HM in the field, you have to manually select the pokemon in order to use it.