Personally, while I like the other games in the series a lot, I just can't knock MGS 1 off my own personal top spot. I was 14 when the game was released, I remember getting my hands on a copy and being absolutely spellbound with it.This was something so different from everything else that came before that it left such a huge impression on me, the iconic characters, the ingenious puzzles (codec frequencies, heating/cooling the key), the amazing amount of detail for a game at that time, it was a step above everything else. So many moments have stuck with me over the years, the Vulcan Raven fights, the Liquid Snake reveal, the conversations with your support team.
The story was complex but not crazy, the characters were interesting and distinctive, it's just a really well crafted game, and the fantastic low-poly (by today's standards) stick in my mind as a masterpiece of design. Later games improved the graphics, added more to the story, but as someone who was used to playing Mario on the NES and Sonic on the Mega Drive, this was a huge game-changer. I just can't rate the games objectively with the weight of that experience overshadowing the sequels. Having said that I enjoyed the "twist" of MGS 2 and what it was trying to say, MGS 3 does some fantastic work with story, and there are moments in 4 which really play on the emotions of the player in a fantastic way.
Edit: I spent so long writing this the conversation has moved on, ha.
