As Montana says it's all about personal taste, however since you asked:
Obviously the big 3: Zelda, Mario, Metroid - all great in their own right
RE4
Zack & Wiki (or so everyone says, havn't played it)
Godfather (some will go WTF, but any Wii owner who has this will tell you the use of the Wiimote is fantastic and makes this good game great) Scarface is also really good, but I prefer GF
WiiPlay (might as well, you'll need another Wiimote and it's only $10 more)
Plus you need a party game or two:
I suggest Rayman Raving Rabbids - it makes great use of the Wiimote, is a lot of fun and only $30. I'll get RRR2 when it drops in price too. RRR2 is better multiplayer, RRR better single player
MP8 - I hate to suggest it, but there's no denying that non-videogaming friends/family love this game.
Mario & Sonic is a lot of fun too in small dosages
Mario Strikers - crazed fun, not good for non-gamers, great online play.
My son only recently got a DS so all I can suggest is:
Zelda
Mario Kart