I'll start off with a few:
StreetpassBest: New Super Mario Bros. 2. Receiving someone's streetpass, and trying to beat their coin record for a certain set of three stages is quite addictive and rewarding. Super Mario 3D Land hinted at the greatness of this type of thing, but NSMB2 takes it to another level. Mario Tennis Open also gets honorable mention here.
Worst: Resident Evil Revelations. Streetpass gives you small amounts of ammo, like a handgun bullet or a green herb. Not that useful especially when ammo is quite plentiful in the game.
SpotpassBest: Mario Kart 7. Downloading ghosts automatically, and updating for a different set of courses every day. It's pretty awesome for time trial junkies.
Worst: New Super Mario Bros. 2. "Hey, wouldn't you like to know that one BILLION coins have been collected worldwide!" Uh, no, thanks, I'd rather have coin challenges, leaderboards, or something actually useful.
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