Metroid Prime 1 and 2 (get the GC versions, or you can get the
Wii Metroid Prime Trilogy if you feel like trying to get a Wiimote working with Dolphin), MP1 has a widescreen ROM mod for the UI. Windwaker and Twilight Princess are must plays for any Zelda fan. Star Fox Adventures is a good Zelda-like game as well, even if you're not exactly a Star Fox fan (you need a bit of a beefy rig to emulate this one, and it doesn't emulate 100% perfect, more like 98%). You can also get both the Ocarina of Time Multi Pack disc and the Zelda Collector's Disc if you feel like running an emulator in your emulator
.Mario Kart: Double Dash is a lot of fun (I like it more than the Wii MK a lot of the time), and there are a couple good Need For Speed games as well. Fire Emblem: Path of Radiance is a good strategy title, and Tales of Symphonia is an excellent RPG... What else... Oh, Kirby Air Ride may seem like a simple game on the outside, but I've poured hours of my life into it. It's a lot of fun, especially if you have at least one other person to play with. Star Fox Assault is a pretty solid entry in the SF series if you're a fan, and the game benefits from HD. It's also one of the easiest GC games to emulate. F-Zero GX is a good game... Uhm... Pikmin 1 and 2 for all your Pikmin needs. Soul Calibur II is a solid fighting game that still looks fantastic, HD really makes it shine. Plus Link is the guest character on that version, and he kicks ass. Pokemon Colosseum and Pokemon XD: Gale of Darkness are interesting entries in the Pokemon series. They show their age, sadly, and some of the development choices are odd, but still fun games. If you're a fan of the old defunct Zoids TV show, Zoids: Battle Legends isn't a terrible way to spend a few hours. I've certainly played worse games. The last one I can think of right now is another niche game, but still something you could dump a good chunk of time into. Custom Robo isn't terribly mainstream, but if you like top-down tactical shooters with absurd weapons and a unique feel, this one is fun.