Any game by Nintendo, Sega, Capcom, Ubi Soft or Namco is worth trying.
Not Star Fox. I'm dying for Nintendo to realize that the space-shooter aspects are what made the series good in the past...
Anyway, I don't know what you [the topic poster] already have, so I'll just list whatever...
Super Smash Bros. Melee - If you don't already have it, I don't know what's wrong with you--but I'm listing it because, again, I have no clue what you do or don't own.
Tales of Symphonia - Only non-Pokemon RPG I've really loved since Star Ocean 2 on the PS1. Fun battles, great music, great storyline and characters, and no load times. Only $20 by now, too.
Fire Emblem - It's not out yet, but it will be by the time you read this if you're really offline for a few days. GET IT.
F-Zero GX - Fantastic as a single-player racing game, as long as you're not the type who gets angry about a challenge. (If you are, stay far, far away--this is seriously a tough game, which is what makes it so rewarding.) Multiplayer is kind of weak unless you find someone equally as skilled as you who
also owns the game and knows the courses, but still, this is absolutely a must-have for some really impressive gameplay, graphics, music, and presentation. Another $20 game, too.
Metroid Prime - Only great first-person platforming in history. (And that includes the sequel, which wasn't nearly as good.) Yet again, just $20.
Those are the must-owns for everybody IMO, while everything else is pretty optional depending on your tastes. (And this is coming from a GC-only owner. I did buy a PS1 and might get a PS3 if it's good enough, but PS2 didn't live up to its predecessor as far as being an RPG system.)
Metroid Prime 2, Paper Mario 2, Mario Sunshine, Wind Waker, Soul Calibur 2, Pokemon XD, Mario Kart, Donkey Kong: Jungle Beat--they're all at least in the 8.0 category as far as I'm concerned, with Soul Calibur probably the most important of them, but not 9.0+ necessities unless you liked previous entries in the series. For example, for me personally, I'd still classify Mario, Zelda, and Pokemon as must-haves because the core gameplay is still so solid even if the in-game areas are lacking, but meanwhile Paper Mario 2 and Mario Kart are $20-or-bust games, and Metroid Prime 2 would have been if I'd known in advance how far the quality had dropped.
I can't attest to Skies of Arcadia just because I haven't been able to find a sealed copy for $20, but a lot of others say it's good, so there's that. Ditto on Viewtiful Joe.
Also, if you liked the Smackdown series, you might be interested in Day of Reckoning or DoR2. I personally can't stand that sort of ridiculous, unrealistic gameplay (ha, I roll out of the way of your stomp and instantly get back to my feet two seconds after getting Stunnered), but some people like it, so... yeah. The thought of giving more money to THQ at this point sickens me, but whatever works for some people.
Last note: I don't know how you could be interested in Mario Tennis, but not Mario Strikers.
