Pikachu

Here he is, folks, the golden calf of the Pokemon brand. Pikachu was chosen relatively last-minute to be the "mascot" monster of the series, and twenty years later, the world generally ranges from marginally indifferent to positively sick of the yellow rodent. This is an inevitability for any single character saturating the market to such an extent, so it's hardly pika's fault, and they honestly made a pretty good choice here. Pikachu is simple to draw, easy to recognize at a glance, just cute enough - especially in these blobbier Sugimori drawings - and just slightly weirder than a normal, mundane animal thanks to its cartoon lightning-bolt tail and electrical powers.

It really does feel like the most "marketable" critter in the first generation; a safe bet to gamble so much brand recognition on, and one that ultimately paid off.