The Hexagon Brothers in Pokémon XD always make me laugh ridiculously hard when they appear and try a count off on who is present.
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Gets me every single time, especially later when they re-appear and count off properly!
But yes, there are certain random trainers who stick in peoples minds... Joey, Shorts guy, Old Man, Earl (The teacher dude), the Magma/Aqua grunts etc... There is some quality ones... OH the foreign team rocket guy who we rediscover in Unova!