I would agree that pristine versions of the two white ones probably top the list. If not for that factor, it's hard to say. In my case, the first and easiest one for me to find was Sea Breeze (my first ebay buy 19 years ago for $5) but her shell is still the only one I don't have, so I have no idea what that says about her rarity. Somewhere there must be a shell that is missing its pony, because logic dictates there should be equal numbers of shells and ponies out there...
They weren't sold here so it's hard for me to judge on the ground rarity. I would agree High Tide being one of the easier ones, but that only because hers is the only box I have managed to be able to scrabble together...but then there's the pink hair issue. Come to think of it, Wave Jumper and High Tide have potential fading issues that may impact their value.
In conclusion...really hard to know but I think those that have common flaws are probably top of the list if in good condition without those flaws.