I love Green Greens. I think TOs around here are banning it more largely because of some shenanigans I've pulled here which aren't really unfair but are really powerful (definitely at least 60-40 G&W wins against Meta Knight, very strong counterpick that puts MK at a big disadvantage).
Anyway, Green Greens is the hands down best counterpick for Mr. Game & Watch in my opinion, and for those who have a chance, I'd like to explain a bit more clearly how to exploit it.
The strategy to win is a defensive one, and it's all about where you are protected. Mr. Game & Watch has an invincible up special that basically makes him unstoppable as he moves upward; overall the only things that are really safe to come right down on him predictably with are things like Kirby's Stone (which tend to have other issues that make it not so great to rely on a lot). The blocks form a wall that tends to be pretty hard to move over as well. Put it together. Just stand on a side platform (when you have a lead) and wait. If they come over the blocks, use Fire, hit them, and run to the other side and wait there. This works against all characters, though characters who don't have any really special way to hit below them (something to throw or something like Stone) and who have poor horizontal air speed have the worst time dealing with it. Guys like Meta Knight and King Dedede have major issues here to this.
This is obviously not instant win so I'll go into how to stop it. Some characters simply camp back and better. Snake is the easiest case; he can just blow up the blocks with c4 and grenades and then keep raining explosives on you. Olimar can use his fsmash to clear out blocks and then rain pikmin on you, and he can be clever with how pikmin are kinda glitchy with the blocks (some colors get stuck on them via pikmin toss) to be really obnoxious. Both Links, Peach, and Diddy Kong to an extent can rain stuff on you from above (Peach, alternatively, could cycle turnips until she gets a beam sword which is so disjointed it will let her take out a wall of blocks super fast).
In general though, you just have to try to destroy the walls of blocks efficiently enough to approach from the side. This is still not bad for Mr. Game & Watch because he can use the opportunity to sneak in turtle or fair, and he can play defensively while the blocks respawn (it's pretty fast). I'm not convinced it's super overpowered, but it's very good, especially in some matchups.
Also, a last bit, if you're down as Mr. Game & Watch and they are camping the sides, you can easily destroy the blocks and pressure then with Chef. The platforms are lined up really well for that sort of trickery. Do watch out for the wind though; it can blow you into dropping bomb blocks if you are careless. You might not force a good opponent to approach just because of Chef, but you will force them back so you can safely key their shield (shield pushback will put them on the ledge or off-stage).
Camping aside, the stage is also good for Mr. Game & Watch because it's so small that his smashes kill super low, and he can have some fun on the ledges if he wants. It's pretty easy for him to detonate bomb blocks safely with his huge disjoints, and the apples are just generally fun for him.
This is also a general argument for why King Dedede really doesn't make the stage ban-worthy. If a DDD player picks this stage and the opponent goes Mr. Game & Watch, it sucks big time for him (he just hard CP'd himself). I also do generally feel that this stage isn't just "comparatively bad" for Meta Knight but actually "really quite terrible" for him. His type of mobility just isn't what this stage rewards at all.
Anyway, Green Greens is the hands down best counterpick for Mr. Game & Watch in my opinion, and for those who have a chance, I'd like to explain a bit more clearly how to exploit it.
The strategy to win is a defensive one, and it's all about where you are protected. Mr. Game & Watch has an invincible up special that basically makes him unstoppable as he moves upward; overall the only things that are really safe to come right down on him predictably with are things like Kirby's Stone (which tend to have other issues that make it not so great to rely on a lot). The blocks form a wall that tends to be pretty hard to move over as well. Put it together. Just stand on a side platform (when you have a lead) and wait. If they come over the blocks, use Fire, hit them, and run to the other side and wait there. This works against all characters, though characters who don't have any really special way to hit below them (something to throw or something like Stone) and who have poor horizontal air speed have the worst time dealing with it. Guys like Meta Knight and King Dedede have major issues here to this.
This is obviously not instant win so I'll go into how to stop it. Some characters simply camp back and better. Snake is the easiest case; he can just blow up the blocks with c4 and grenades and then keep raining explosives on you. Olimar can use his fsmash to clear out blocks and then rain pikmin on you, and he can be clever with how pikmin are kinda glitchy with the blocks (some colors get stuck on them via pikmin toss) to be really obnoxious. Both Links, Peach, and Diddy Kong to an extent can rain stuff on you from above (Peach, alternatively, could cycle turnips until she gets a beam sword which is so disjointed it will let her take out a wall of blocks super fast).
In general though, you just have to try to destroy the walls of blocks efficiently enough to approach from the side. This is still not bad for Mr. Game & Watch because he can use the opportunity to sneak in turtle or fair, and he can play defensively while the blocks respawn (it's pretty fast). I'm not convinced it's super overpowered, but it's very good, especially in some matchups.
Also, a last bit, if you're down as Mr. Game & Watch and they are camping the sides, you can easily destroy the blocks and pressure then with Chef. The platforms are lined up really well for that sort of trickery. Do watch out for the wind though; it can blow you into dropping bomb blocks if you are careless. You might not force a good opponent to approach just because of Chef, but you will force them back so you can safely key their shield (shield pushback will put them on the ledge or off-stage).
Camping aside, the stage is also good for Mr. Game & Watch because it's so small that his smashes kill super low, and he can have some fun on the ledges if he wants. It's pretty easy for him to detonate bomb blocks safely with his huge disjoints, and the apples are just generally fun for him.
This is also a general argument for why King Dedede really doesn't make the stage ban-worthy. If a DDD player picks this stage and the opponent goes Mr. Game & Watch, it sucks big time for him (he just hard CP'd himself). I also do generally feel that this stage isn't just "comparatively bad" for Meta Knight but actually "really quite terrible" for him. His type of mobility just isn't what this stage rewards at all.