Loaded Deck is an absolute must. It really does make Random Deal actually worth the bother, since providing more than one ability is awaiting recast (Two-hours do apply to this calculation, though they will never be reset) it will
always work, no other strings attached whatsoever. When combined with the Commodore Frac, this has a chance to reset two abilities rather than one.
The rest is really personal preference to your playing style, or what you use your Corsair for. With regards to group one merits, there is really no "right or wrong" combination; in given circumstances, they're all acceptable and useful. Here's my opinions:
Group OnePhantom Roll Recast:Useful when you need to get rolls up as quickly as possible (arriving to an HNM holding party, something like that), but really lets you down when you're in more long term events. For everyday use, don't really see this being worth it. I upgraded this to 5/5 because of how often I play Corsair. I believe it just lets me function more efficiently. This one is certainly personal preference, though.
Quick Draw Recast:An absolute must. Quick Draw remains one of the one things that makes COR so unique and practical; even if you're just spamming elemental shots for damage, it's practical and helpful to your party or linkshell's situation. This was the first thing I capped in Corsair merits and I've never regretted it. It really is the "ultimate" upgrade anyone can make, in my opinion.
Quick Draw Accuracy:Obviously has its uses. This is also very helpful if you're a Quick Draw spamming Corsair, since the more accurate they are, the more damage they do or the more they'll stick. Elemental Quick Draws don't miss, though, so this really only directly affects Light Shot. More accuracy will mean your elemental quick draws will hit top damage more often, but I don't consider five accuracy that significant. At 75, stacking the Agility on is pretty straight forward. Light Shots should not be missing much, and if it's missing continuously, you're probably in a situation where you shouldn't be relying on it anyway.
Really accurate Light Shots are fantastic, they will save your life. I've even slept Behemoth before with Light Shot upon claim. This will generally help your overall DPS with elemental draws as well, since it will allow you to hit the top end of damage more often.
Random Deal Recast:Not really worth it in my opinion. Even when stacked with Loaded Deck, I don't really consider this worth it. The only time I "rely" on Random Deal is to reset Convert in merit parties. Outside of this, I just use it when I have it and never really rely on it for any reason.
Bust Duration:Anyone busting a lot is playing the job wrong. The important rolls should not be doubled up on 6 or above if you
need the rolls up (Magus's Roll on Ultima, for example). I believe this makes this category a waste of merits. If you have Fold, which you do, it makes it even more pointless. More than one bust every five minutes would be pretty bad luck if you're careful with rolling.
Group TwoSnake Eye:An absolute must for any Corsair. Whether it's rolling off of an unlucky number, rolling up to eleven or rolling the lucky number, Snake Eye is fantastic. I didn't personally go 5/5 on this, though I would've done if I had more upgrades available. Since I got Loaded Deck and Fold as well, I didn't have space to upgrade this to full and Winning Streak;
Fold:Very useful, but not worth merriting off of level one in my opinion. Busts should not be common enough to make spending further merits on this worthwhile.
Winning Streak:I went 5/5 on this and I don't regret it whatsoever. Possibly the best part of Corsair merits I upgraded. Fully upgraded, it makes rolls last for 6 minutes 40 seconds; this is fantastic when you roll an 11 and want to milk it as much as possible. Also exceptionally useful if you're doing a long term event where people aren't always stationary. On the rare occasion they are in a group, making the roll count is essential. The extended duration is really, truly fantastic. I can say that as somebody who's been COR to just about everything; Dynamis, HNM, Sea, Limbus, Einherjar, Salvage, etc.
Loaded Deck:Level one is essential. It means that if more than one ability is waiting on recast, it will never fail to rest an ability. More than level one isn't worth it in my opinion; if 5/5 disabled the chance of failure (when only one ability is waiting on recast), I'd probably have gone for it. Sadly, it doesn't, so I left this at level 1 just for the two or more ability guarantee.
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As I said, it's really about preference. What works for me might not work for everyone else. I more or less live on Corsair, however, and I'm very happy with my merit choices.