Streetpass Best: 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.
Spotpass Best: 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.
Spotpass for Kid Icarus gives you a new weapon gem daily. It's a nice touch at the start anyway, but as you play through the game, you'll end up with more powerful stuff on your own.
Streetpass also has the potential to be great, but it comes down to the people using it. I use it to share copies of my most powerful weapons. In return I get sub-200 rated junk from someone who's average intensity is 2.0. Grr.
I'd say the game makes better use of the features than most titles though.
@Shadowlink I actually find the streetpass in Kid Icarus kinda lackluster, because the gems you get from strangers require a SHIT TON of hearts to convert. Like, a ridiculous amount. Unless the weapon is shitty. And you still can't collect weapons that you haven't unlocked yourself, that would have been nice. It is nice that you can see the player's game stats, though.
The spotpass is cool, and affordable at least.
@GameDadGrant Mario Kart 7 streetpass is nice, but I find everyone sends me just the Toad Circuit time trial, which gets boring to replay. The grand prix is kinda cool, but I've never used the "meet up online" feature.