The worst in the series to me, by far. The cutscenes are boring and far too long for how uninteresting they are. The story itself is garbage that came from Kojima listening to his insane fan base that is just over-explanation city. Not as inventive as MGS2 and not fun like MGS3. The characters are mostly terrible, but again, not in a fun way like previous games (any cutscene with Naomi is enough to make my eyes roll backwards into my head). Marked the turning point where the fan service in the games went from a bit much to extremely creepy. Let's fetishize PTSD-stricken war victims who will dance and pose for you- wooooo!
Most importantly, the meager amount of gameplay in there is clunky and poorly balanced after the initial spectacle of it wears off. The war areas are half-baked and more annoying than anything with things like infinite spawning enemies in a stealth game, straight out of CoD. Can be sneaking through an area and a enemy will just appear out of nowhere and run straight into you. The problems with chapter 3 and 4 are well established so I won't even get into them. I feel there's only five truly good sneaking sequences in the entire game: the start of chapter 1 until you get your gear from MK2, infiltrating liquids camp after you see the beasts the first time, The very beginning village of chapter 2, tracking trail after laughing octopus, and the one sneaking area in chapter 5. The rest are all marred by other crap that distracts and annoys (see infinite respawns, etc).
Oh and the shooting feels clunky, enemies aren't fun to shoot, they barely react. They just twitch as you shoot loads of bullets into them until they do their ridiculous ragdoll somersault upon death. Just a huge downgrade from previous games. Which sucks given the game wants you shoot those guns during the war sequences. The large amount of weapons feel like a squandered opportunity given how little you'll use them. Oh and another reason the shooting action feels unsatisfying, Snake constantly stumbles with hit stun as he's shot taking away his aim, it doesn't increase the difficulty, it just makes the action feel tedious and janky. Previous games had a bit of this but it was rarer, and they didn't focus on the action as much as this one.
The boss fights are fun overall, but have some serious issues too. The same stupid post-robot boss fight with the beasts wears out its welcome after the first one. It just straight up repeats and later ones add nothing new. Just an excuse to ogle the aforementioned women. The bosses themselves are a spectacle, but outside of maybe the wolf fight there's next to no variety in how you fight them, just big ol' damage sponges. A total letdown after MGS3. Heck, even for all the complaints about the bosses in MGSV at least you had other options with how you dealt with them rather than just shoot until dead/knocked out.
Basically the game can be neat the first time through, and certainly brings a lot of spectacle and nostalgia. However, after that, it just doesn't hold up unlike the rest of the games. Replays feel like a complete chore. If you did like the game, you're doing yourself a favor if you never replay it.