only played a bit of metal gear solid 2 & none of three, four, or however many future/flashback ones there are now, but metal gear solid for ps was probably one of my favourite games of all time.. beat the whole thing at least 3-4 times & unlocked a lot of the secrets:
forget which ending scenario gets you which bonus item (you start with at the next beginning of the game), but it depends on whether meryl survives with you - bandana or invisibility (you obviously want to beat the game at least twice so you can breeze through it once with both items)
think after beating it 3 times, you get to play it over with snake wearing a tuxedo
theres also ghosts hidden in the game you can flick with the camera (only shows up after if you flick the right spots)
psycho mantis (boss with the gas mask who flies & has psychic powers) cant read your controller ("mind") if you switch slots during his encounter. tried this, but idk if it made too much of a difference
while running through the areas with wolves, they wont bite you if your item is set to grey wolf or whatevers handkerchief. then they just make cute sounds & have a little heart icon floating above them
more but cant remember
the graphics are so shit compared to the other ones of course, but it was one of those double-disc games at the time with a lot of work put into it. came out well