Ok my turn:
Harvest
Hands down best ability for this CAP! I don't agree with anyone who says that this ability is too niche or too specific for a Pokemon and so the Pokemon must be designed for it to work. We are talking about one of the most versatile abilities ever, so how can a reasoning like this be right? Here the things that Harvest will bring to the table:
- Rest + Lum Berry to have reliable recovery and status immunity in order to succesfully take on the water-types found on rain teams, enabling the CAP to succesfully take one of the roles we want it to.
- Sitrus Berry with some decent special bulk (90 HP + 90 SpD) will allow the CAP to take on even 2 Specs DM from Latios and threaten it back. However we don't want from the CAP to be able to switch with impunity into Latios, so it is good that Latios can still fuck it over with Sun-boosted HP Fire or Trick. This will help the CAP to succeed in one if its main roles, checking Lati@s.
- Chople Berry to better take and Keldeo (significant rain threat) and Fighting attacks in general. Notice how the CAP will be still threatened by Keldeo as Secred Sword should still hurt even when hitting neutrally as imo we shouldn't make the CAP very physically bulky. Not only this but the CAP won't have lefties meaning that it will be easy to wear down when holding a Chople Berry.
- Harvest is an ability meant to work in sun. It is usable outside of it but maybe not reliable enough for OU standards, which will discourage the use of it outside of sun, which as already mentioned will be a good thing.
- Finally, there will be other usable ways to abuse Harvest that will make it a bit more versatile, which will make sun teams more versatile to an extend, a feature they really need.
The only real downside with Harvest would be the potential to make the CAP too good, as Harvest is so versatile and good, and we have never seen a decent Pokemon with it, meaning that we don't know the full extend of its potential and power. But i believe that with carefull thinking and correct decisions we will be able to prevent this from happening. And anyway what could be more fun and educative from exploring the potential of one of the most intriguing and creative abilities?
Chlorophyll
I am not a fan of this one and here are my reasons... First, it doesn't help the CAP at all against Water-types. Let's not forget that one of the CAP's main roles would be to act as a pivot for Politoed, meaning that the majority of the time the CAP will be on the field, it will be raining, making Cholrophyll absolutely useless. Second, it is not reliable at all for taking on Latios. Yeah it enables the CAP to outspeed Latios,
IN SUN ONLY, but this doesn't automatically mean that it beats it. Assuming that the CAP will get Pursuit and will use another Dark STAB move with good BP, the CAP still has to play a 50-50 mind game in order to kill Latios. So for example Latios uses DM as CAP switches in, CAP loses most of its health, and then next turn CAP has to predict correctly in order to eliminate Latios, and if it fails it dies while Latios is still alive and able to outspeed almost the whole sun team while threatening to OHKO everything.
So the conclusion is that Chorophyll is not reliable because sun won't always be up, and unlike Venusaur which is a sweeper, CAP will be expected to come in when opposing weather inducers are still alive, while also not helping a lot against the threats that we want the CAP to handle. Finally, Chlorophyll is a very troubling ability for weatherless teams, which already have Venusaur to worry about. So my point is, how will weatherless teams be able to compete against two Pokemon that outspeed almost everything while being able to switch moves and hold items? This means that Chlorophyll not only doesn't help a lot with the initial goal of the CAP, it also makes it stronger in other aspects, which we don't care or don't want it to.
Does handicaping weatherless teams against sun teams even more sound like a good idea?
Flower Gift
This one is tricky... When this ability is active it is the best thing even for the CAP, helping it with Water-types and Lati@s, but it won't always be active, so the problem lies on its reliability. Against rain teams it won't be so useful, as the CAP is supposed to work as the main switch-in for Politoed, meaning that rain will be up while the CAP is on the field the majority of the time. Against Lati@s it depends on the kind of team we are facing and it is too complex to analyze so i am leaving it as it is. So my main problem with this ability is that we can't really tell if it will be reliable enough to work, but if it ends up being reliable enough the benefits will be huge, so consider it something like a big risk-big reward kind of thing.
Natural Cure
This is an ability that helps directly the CAP to deal with the threats we want it to, namely Water and Electric types (Scald, Toxic, and Thunder) which is a good thing. However, is this enough? Only if for some reason we don't chose better abilities that exist right now, such as Harvest and Regenerator.
Regenerator
This ability is the shit, as we all know by now! There is no doubt that this one of the best abilities, not only for this Pokemon, but for any Pokemon in general. It will certainly allow the CAP to better stomach hits from water-types and stay healthy without giving free turns, and it will also help it against Lati@s, allowing it to act as a pivot for strong DM's, tanking the intitial strong hit, so that the next turn another teammate will be able to take the weaker hit. It's not like the CAP should be able to hold back Lati@s for an extended period of time, as most sun teams are offensive by nature. However, my main problem with this ability is that it will not only work for sun teams, it will work for any kind of team that benefits from the CAP's positive qualities. Furthermore, even though it certainly helps CAP to deal with the threats we want it to, i feel that Harvest helps more, so there is that too.
Cloud Nine and Water Absorb / Storm Drain
I don't see how these will help at all. The CAP's main problem against water-types will be Ice Beam and status from Toxic and Scald, not powerful water moves, as those can be handled with a decent stat spread anyway, while the issues i mentioned cannot. Also both abilities don't help at all the CAP with Lati@s. So useless abilities overall. Not to mention that with Cloud Nine the CAP will have potential to be used in Sand and weatherless teams to outspeed and threaten Venusaur (with or without Scarf depending on its Speed) which is clearly not a positive thing for sun teams.
Thick Fat
Good ability. Helps against Ice Beam as well as boosted HP Fire from Lati@s. However it faces the same problem with Natural Cure, it just isn't useful enough and should be picked only if the superior abilities are out of the way for some reason.
Leaf Guard and Solar Power
If Chlorophyll was bad, these are godawfull, and don't help the CAP with its roles at all, simple as that...
You can assume that any ability i didn't talk about from the abilities that are already mentioned in this thread was just not worth a mention.
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So to sum it up i feel that Harvest, Regenerator, and Flower Gift are the best abilities for the CAP, in order of preferance.
EDIT: Oh and something else about Harvest... For those who think that Harvest doesn't fit with the concept of the CAP, meaning the Pokemon with niche roles that is not supposed to do a lot of things in general, i have to say this. Even with Harvest the CAP will still have surefire counters and ways to BE dealt with it for two reasons. First, it has a set of crippling weaknesses, making it easy to prepare for even when hoding a type-resist berry. No matter what you do this CAP will always be murdered by Fire and Bug moves. Second, we already have chosen what we want to counter or check our CAP, and this won't change in the ability selection stage. Even if we pick Harvest the CAP will still threatened or countered by Fire, Steel, Bug, and some Fighting types. So even with Harvest the role of the CAP will be specific enough to do only what we want it to.