I decided to drop the PSP from this article because, frankly, I know nothing about it. For the record I’d be interested in WTF and the Metal Gear games on that system and that’s about it.
The Wii and DS operate in a completely different stratosphere when compared to the 360 and PS3. Yes, the PS3 price drop has helped a lot, especially in Japan, but Nintendo’s consoles just appeal to such a wide variety of gamers (young and old, casual and hardcore, male and female) that it’s tough to break down what really sells systems. But hey, I’ll give it a shot.
Nintendo Wii

System Seller: Wii anything
I was tempted to put the Wii itself as its own system seller, because that’s pretty much true. The Wii moves so many units because of all they hype generated around it. People gravitate to all the “Wii” games, Wii Fit sells a lot of systems, but many people are just content getting the Wii for Wii Sports and Wii Sports Resort. Even Wii Play sells tons of units, although that isn’t really a system seller as much as it is something that people pick up when they get a Wii (for the extra controller or whatever). The Wii, unlike any other gaming system, sells to the average consumer with its accessibility, which is why it is destroying the competition.

Must Haves: The Legend of Zelda: Twilight Princess, Metroid Prime: Trilogy, Super Mario Galaxy, Super Smash Brothers: Brawl, Wii Sports: Resort (or equivalent)
The sad thing about this list is that two of the games came out around launch (Trilogy is all the Metroid Prime games with the new Wii controls though, so I guess it’s technically new) and Galaxy came out like 2 years ago. For serious gamers, there’s not a lot to offer on the Wii, especially 3rd party content. That being said, Zelda and Mario, like all Nintendo first party games, are excellent, and Metroid Prime is one of my favorite video game series ever. Metroid also uses the controller very effectively and is the best first person experience I’ve had on the Wii. I threw Wii Sports: Resort on there because if you own a Wii it’s almost mandatory that you own a good party game, but really there are a lot of choices out there: Boom Blox is pretty fun - you could even get away with a game like Warioware: Smooth Moves, and of course almost everyone owns Mario Kart (just check the NPDs). Thankfully, Nintendo is releasing a couple of new Mario games this year and next, so hopefully there will be a reason for me to dust off my Wii soon.
Upcoming: New Super Mario Bros., Super Mario Galaxy 2, Wii Fit Plus
Nintendo DS

System Seller: Pokemon (kids), Brain Age (adults)
It’s no surprise that the average owner of a handheld console is much younger than the average owner of a home console. What better way to keep the kids occupied on long car trips than have them “catch them all”? I played Pokemon on the GBA, and it was a surprisingly deep game, and according to reviews the DS versions are still decent. Regardless, Pokemon probably sells the most DSes to kids, whereas older adults, including very old adults (like my parents) like the DS for games like Brain Age, simple brain exercises that people like to play to keep themselves young. If you’re old, you buy the DS, if you’re young, you buy the DS. No wonder it’s sold the most units in the current generation.

Must Haves: Square-Enix RPGs, Professor Layton series, Phoenix Wright series, Elite Beat Agents (Ouendan in Japan)
The crazy thing about the Nintendo DS is that there is something for everyone. For the most part, the “casual” stuff sells systems, and that’s what a lot of people focus on. But if you like old-school RPGs, there are the re-releases of Dragon Quest and Final Fantasy, for people who like puzzles, there is Professor Layton, Phoenix Wright tickles one’s adventure fancy, and Elite Beat Agents is for rhythm game fanatics. And if none of those genres floats your boat, there are legions of other game types out there (I’m sure I’ve forgotten a bunch of excellent games). As espion4ge has stated elsewhere, the DS really is one of the best values in gaming and a great investment for any gamer, especially if you’re on the go. If you spend time shopping with your wife (check), take public transportation (check), or like to play games before you drop off to sleep (now outlawed in my household), the DS is a great buy. Just do yourself a favor and go with the Lite over the DSi, so you’ll save yourself some money.
Upcoming: The Legend of Zelda: Spirit Tracks, Ace Attorney: Miles Edgeworth
So that’s my take on the best games out there for the current systems. Of course, most of it is based around my preferences, so I’d love to hear what games you all love to play on your systems! Especially if it’s the PSP…haha.