I would go PS1 for the best all around experience and nostalgia but it really depends on what you want now that that generation is long past.

The Saturn excels at its huge import library which is full of absolutely gorgeous 2D games along with great SHUMPs.

The N64 has what many would consider the definitive 3D entries of the various Nintendo properties along with some great third party games.

The PS1 has the best variety of software from RPGs, Platformers, FPS, etc. of the time, had the best third party support of all the systems too.

Rough Selection of games off the top of my head I enjoyed back when they were new along with a few recent discoveries:

Crash 1/2/3
Sypro 1/2/3
Grand Turismo 1/2
Final Fantasy 7/8 (haven't played 9, heard great things though)
Metal Gear Solid
Resident Evil 1/2/3
Parasite Eve 1/2
Final Fantasy Tactics
Chrono Cross
Twisted Metal 1/2/3
Meal of Honor 1/2
Cool Boarders 1/2/3
Tony Hawk's Pro Skater 1/2
SaGA Frontier
Xenogears
Armored Core Master's of Arena
Syphon Filter
Raystorm
Fear Effect 1/2
Jet Moto 1/2