Personally, for best spinoff I'd go with Colosseum. It uses the core gameplay mechanics, but with additions that emphasize the need to raise your critters rather than simply let them rot in the PC. Also features pretty much the only hero of the RPGs who can be taken remotely seriously (read: is not a 10-year-old n00b who upstages all the experienced trainers).
A shame Gale of Darkness went the wrong way in attempting to expand it. They added tons more Shadows, mandating an "automated" method of purification, which rendered the whole idea of bonding with your Pokemon to purify them obsolete. What they need is to take Colosseum's level of Shadows (40-something, I believe) and attach it to a game that has normal wild encounters.
Snap was a lot of fun, though.
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