Anyone who gathers at end game knows how valuable these things are.
Disclaimer: I am assuming level 60 DoL here, with around 650-700 GP. Also, I am ignoring leves and quests as a source, since they can't really provide cordials in any significant quantity.
So let's look at how each of the GP restorers rate:
Watered Cordial (NQ)
GP: 150, Cooldown: 140s, GP/s: 1.07, Acquire: crafted (quick synthesis).
Cheap and easy to make, these only require one ingredient from a time-gated botany node. However, the paltry amount of GP makes it hardly worth it.
Overall rating: 3/10
Watered Cordial (HQ)
GP: 200, Cooldown: 126s, GP/s: 1.59, Acquire: crafted (manual).
Cheap and easy (although time consuming) to make. At first glance, these seem like a good choice. In practice, you need to be chugging them like a freshman at frat kegger. It also means they take up more inventory space.
Overall rating: 5/10
Cordial (NQ)
GP: 300, Cooldown: 240s, GP/s: 1.25, Acquire: GC Seals, crafted (quick synthesis).
Super easy to get in quantity. Craft via quick synth, or just head down to your GC pusher and score some. The only real downside to these is the long cooldown. 4 minutes is an eternity when most of the windows are spaced less than that. You end up having to decide which nodes to ignore when faced with a series of closely timed pops.
Overall rating: 6/10
Cordial (HQ)
GP: 350, Cooldown: 216s, GP/s: 1.62, Acquire: crafted (manual).
Wow. These things are a royal pain in the butt to get, and time consuming to make. The extra 50 GP and 24s shorter cooldown makes a massive difference though. They are almost as good as Hi-Cordials. At 350 GP, they give enough of a boost to hit mutliple nodes, sometimes in the same window. These are my top choice for any sustained gathering effort.
Overall rating: 7/10
Hi-Cordial
GP: 400, Cooldown: 180s, GP/s: 2.22, Acquire: 33 blue gathering scrip.
Hands down, best stats in the game (so far), but not by a whole lot. They're blue scrip vendor only (and not cheap either), though, which is a complete deal breaker for me.
Overall rating: 2/10