Now that I've had time to think about it, here's what I think of all the main series Metal Gear Solid games besides the newest entry, Phantom Pain.
Metal Gear Solid - 8.5/10
It's a 1998 PS1 game, so my expectations were pretty low with the first game I tried out. I was surprised in the best way imaginable.
It's remarkable how Kojima's vision for a truly great story in a video game was realized this early on, and how he was able to pin-point what he could use as far as what the technology at the time could give him. He was genius enough to realize that fully-voiced characters with well-written dialogue was the absolute best way to get the player emotionally involved in his story when the technology wouldn't allow "real" cut-scenes, and how a character-based tale with genuine conflict throughout could really create something the industry had never really seen before. Excellent voice acting on top of a somewhat philosophical and thought-provoking theme of genetics make this an excellent starting point for the series plot-wise.
The game is no slouch in the actual gameplay department either. Yes, some of the concepts, such as a first-person view, are clunky and not implemented to perfection, but the core concepts of stealth and weapon use make a great transition to 3D. I was somewhat frustrated at some of the more cryptic and "old-school-hard" parts of the game (like certain insta-death traps), but the overall experience is still very enjoyable. I should also mention that some of these bosses are pure genius; Psycho Mantis had me completely confused in the most innovative way possible.
Metal Gear Solid 2: Sons of Liberty - 8/10
I was surprised when I realized that I actually liked the PS1 original better than the PS2 sequel. I think it's mainly because, yes, I don't like how Raiden steals the spotlight from Snake. Yes, it's interesting that we get to see Snake in more of a mentor role, but I feel like this comes at the cost of completely losing our emotional investment in the player character; I just never really felt like Raiden was ever as relate-able or fun as a character as Snake was in the original. I also feel like the plot, while again interesting due to its social commentary undertones, is kind of all over the place. I had a lot of unanswered questions at the end, and not in the good "you have to come to the conclusion yourself" type of way. The supporting cast of characters also isn't as excellent as the previous game. Liquid was a far more interesting antagonist than Solidus, and Rosemary wasn't nearly as well implemented as Naomi when it came to the intrigue elements of the plot. Altogether good, but a step back in a lot of ways.
I also think it's inferior to the original in certain other gameplay aspects. The Big Shell is a bore to explore through, and doesn't have that type of "James Bond" infiltration feel as Shadow Moses did. I do, however, greatly appreciate how the generational shift made the weapons play much more intuitive and fun. The first person aiming is greatly improved, but other things, like swimming, are still very much sloppily implemented. The bosses also kind of take a step back as well - encounters like Fatman and Vamp have absolutely nothing on battles like Psycho Mantis or Cyborg Ninja in the original.
Metal Gear Solid 3: Snake Eater - 9.5/10
Now THIS is a sequel. I almost can't compound all the good things I have to say about this game in just a few paragraphs, but I'll try. The switch in perspective to Big Boss during the Cold War era was an excellent choice, giving us essential backstory to the character that is truthfully the most influential and essential to the entire Metal Gear plot. I adore how we get to see him at his origins, having to take down his mentor and dear friend due to her defecting to the Soviet Union. The parallels it makes to Snake's first major journey in MGS1 is also pulled off incredibly well, and the ending is hands-down one of the best I've ever experienced in a video game. Seeing the origins of the mysterious "Patriots", something that wasn't well explained or thought out in MGS2, is also done in a way that makes complete sense according to what we know about WWII in this universe. Altogether the best plot of a Metal Gear game hand down.
MGS3 is also the first instance where the series fully embraces a 3rd-person perspective instead of the traditional top-down approach, and I think the result is an altogether better experience. The weapons are finally able to be used to their fully potential, with excellent aiming and usage mechanics. The environments are the most varied and interesting in the entire series up to that point, with jungles, rivers, enemy bases, deserts, mountains, all that kind of good stuff. The concept of "survival" is very well done as well, with camo indexes and hunting for food, which makes the experience all that much more believable. I'm also going to go out on a limb and say the bosses are definitely comparable to MGS1 as well; the fight with The End was such an excellent concept and change of pace, that it's hard not to fully appreciate what they were going for.
Metal Gear Solid 4: Guns of the Patriots - 9/10
Assigned the dubious task of tying together the entirety of the series along with Solid Snake's final journey, I think MGS4 does it very, very well. Yes, the cutscenes are probably too long and bloated for their own good. But what I think you have to realize is that we had this enormous cast of characters throughout three games, and Kojima truly did the best he could giving every single one of them a satisfying conclusion that made sense and didn't leave any noticeable holes or gaps in the story. It probably, at its core, isn't AS good as a story as what MGS3 had, but it's still very high up there when it comes to plots that get you involved and invested into the characters and events, especially given the concluding nature of Solid Snake's last journey.
I'll get crucified for saying it, but I think MGS4 has the best mechanics of the series. I love how the "levels" are still open-ended when it comes to the sheer amount of ways you can approach and deal with the situation, and how the level of detail means that you'll truly buy that you're in these realistic and real-world environments. "Octo-camo" is an awesome gadget, and the addition of Drebin and his shop make it feel like you're rewarded for going out of your way to explore and take out as many people as possible. It's a shame that actual gameplay time is almost matched by cutscene watching - this would probably be my favorite so far if I actually spent more time playing the game instead of watching it.