No Guard to A Tier
Almost any poke would become better with a 100% accurate Stone Edge, Fire Blast, Blizzard, Thunder, Hydro Pump, Focus Blast, Dynamicpunch, Zap Cannon, Sleep Powder, Hypnosis and Inferno. Imagine how powerful No Guard Starmie would be with a moveset of Blizzard, Hydro Pump and Thunder, or how great Mew would become with options such as Dynamicpunch and Focus Blast at its disposal. Even No Guard Metagross would be awesome with Dynamicpunch and Meteor Mash getting the accuracy boost from the ability. Don't even get me started on No Guard Tornadus or Kyurem...
I agree for A tier for this one, but it does need to be noted No Guard is a double edge sword; your opponent gets 100% accuracy as well, making attacks like Hypnosis and Will-o-Wisp much more dangerous.
Harvest to A Tier
Harvest is simply an amazing ability! It has so many uses it is crazy!!! First of all every poke could get instant recovery and status immunity with it...! You don't believe me? Ask Exeggutor he knows better! Simply slap a Lum Berry and Rest on your poke and here you have it! Or maybe you would like your boosting sweeper to get multiple Speed boosts when behind a Sub with Salac Berry, without doing anything... Or you could get rid of one of your pokes weaknesses altogether with the appropriate weakness-reducing berry... Anything you want Harvest can do it for you! I would go as far as to call it one of the best A Tier abilities, so don't pls don't underestimate this ability so badly...
Well for the time being the inexistance of pinch berries means we can't really consider them in the tiering. I'm sure they'll be released sometime during B2W2's era, but until then we can't consider them. However there's something to note about the other strategies; Harvest only restores a berry 50% of the time, meaning you're going to end up relying on hax a fair bit. For the Lum berry, that means whether you get to heal or not (or weather you wake up in time) depends on how lucky you are, and with the resist berries repeated hits will probably still take you down (not to mention it basically becomes a lesser version of heatproof that only works 50% of the time). Then there's the issue you have to give up on other, potentially more powerful items to use it.
Still, I see your argument and it does have potential. For Pokemon that commonly run berries it has a lot of potential, and a lot of defensive Pokemon could make great use of the ability's ability to partly eliminate weaknesses and allow the liberal use of Rest. The question is if this is enough benefit for A tier, as it doesn't have quite the magnitude of other A-tier abilities (permanent type immunity, effective 1.5x defense, immunity to or benefit from status, etc.) and relies on luck far more than one would like.
Finally Marvel Scale to A Tier
Like Guts is an amazing ability for an offensive poke, Marvel Scale does the same for defensive pokes, and even for offensive, as it shields them from priority. Many defensive pokes or offensive tanks would love this ability, such as Metagross, Vaporeon, RestTalk Gyarados, Ferrothorn, Jellicent and Latias. Almost any poke likes having his defense boosted, so definitely A-Tier.
Yes they'd appreciate the defense boost sure, but the problem is the cost that comes with it. Unlike Guts, where being on a time isn't necessarily that big a deal since you'll do a lot of damage in that time anyway, defensive Pokemon really can't afford the residual damage UNLESS they have Rest-Talk, which has move compatibility issues and is plain unavailable to most Gen V Pokemon. Yes, being statused with a defense boost is better than being statused without one, but it's not really something you can make game changing use of without a specific set. As I've already said, simply universal benefit isn't enough for the A-tier; there has to be a significant IMPACT that benefit has on a regular basis, or most abilities would be A-tier since most abilities don't have downsides
Compound eyes, Heatproof, Filter, Solid Rock and Victory Star are good B Tier abilities, but i think that are not so good for A Tier and are outclassed.
Just another quick reminder... being outclassed in the tier you're in doesn't matter. It DOES matter if you're outclassed by something in the teir BELOW you, because then you're clearly misplaced, but there's no reason one ability can't exist in the same tier as another with a similar effect of a lower magnitude, as long as that magnitude is still high enough to qualify for the tier. Compoundeyes is a great example of this because it appears totally outclassed by No Guard at first glance, but it allows the same thing; the use of more powerful but less accurate moves with much less risk. It does this WITHOUT No Guard's downside as well. Even though the bonus is smaller, the benefits are still good enough to meet A-tier criteria: just about anything would absolutely love 90%-100% accurate 120BP moves and be much more powerful for it. It has the magnitude, so there's no reason it can't be A-tier itself, even if No Guard does outclass it in some respect. I do agree the others are B-tier, but not because they're outclassed; they don't have the magnitude enough to make it to A-tier on their own merit. It's an important distinction.