@Jimera
I don't really get what you are telling about Harvest. Without Sun up, Harvest activates 50% of the time, which is just fine, as i already mentioned. You don't have to use a Harvest poke in sunlight at all... You could, just to be sure, but it is perfectly viable outside of that, as Exxegutor has proved.
Finally you have a point about Marvel Scale, but i just think we have different definitions of a Tier A ability. You say that for an ability to be Tier A, it has to make the poke significantly better, while i think that it has to be make the poke simply better, in general, while of 'course being better than Tier B abilities, which is what Marvel Scale does.
@Scarfwynaut
No Guard, Compound eyes and Victory Star are similar, but not really identical, and No Guard is clearly the best. Let's see some moves, and the accuracy they get with every one of those abilities (i am listing only moves that have wide enough distribution):
- DynamicPunch and Inferno always hit with NG, have 65% accuracy with CE, and have 55% accuracy with VS
- Sing and GrassWhistle always hit with NG, have 71,5% accuracy with CE, and have 60,5% accuracy with VS
- Hypnosis always hits with NG, has 78% accuracy with CE, and has 66% accuracy with VS
- Thunder, Focus Blast, Hurricane and Blizzard always hit with NG, have 91% accuracy with CE, and have 77% accuracy with VS
- WoW, Sleep Powder and Stun Spore always hit with NG, have 97,% accuracy with CE, and have 82,5% accuracy with VS
- Hydro Pump and Stone Edge always hit with NG, have 104% accuracy with CE, and have 88% accuracy with VS
- Fire Blast and Megahorn always hit with NG, have 110,5% accuracy with CE, and have 93,5% accuracy with VS
Any move with 90% accuracy or more will practically always hit with all 3 abilities, so i won't list those moves.
So what we can see is that there is quite a difference between NG, CE and VS. NG makes viable moves such as Dynamicpunch, Inferno, Hypnosis, Sing and GrassWhistle, were VS does shit, and CE makes them viable too, but they still have very disapointing accuracy.
Any move with 70% accuracy becomes viable with NG, and also becomes viable by CE,which boosts the accuracy to 91%, which is almost perfect, but could still cost you a few games. VS brings those moves to an accuracy level (77%) where they are just barely acceptable in terms of viability standards (i think that almost anyone wouldn't use a move that has lower accuracy than 75%, if he had a choice).
Any move with 75% accuracy or more is practically fully accurate with CE, while with VS, most moves get pumped up to quite acceptable levels.
Now let's see how these abilities are similar or not, by using some real examples. Let's take Torandus which commonly uses Hurricane and Focus Blast. No Guard would be the best ability of 'course but what about the other 2? Would they be way worse, or almost as good? Well CE would be almost as good, giving to those 2 moves 91% acc., so in practice you would rarely feel the difference. But VS would bring those 2 moves to 77% accuracy, which is downright horrible, as almost no poke would want his only STAB move AND his basic coverage move to have such sucky accuracy.
Another example that is the same as the above is Kyurem, which uses Blizzard and Focus Blast.
Anyway we can see that most pokes would be just ok with No Guard and CE (of 'course NG would be the best ability, but CE wouldn't be too far behind), but not with VS. And this is why the first 2 abilities deserve the A Tier status (NG and CE), while the third doesn't (VS).