Wii Sports Resort: This really depends in whether you liked the original Wii Sports of not. If the original was boring, Resort will be boring, but if for you the original was sweet and you still play it once in a while, go ahead: with more sports than the previous one and a free MotionPlus, Resort will be a great investment. Now the game may be more precise, but in the end it's just a different set of sports; I got bored quickly with it, but that's just me.
CoD: Have you played a FPS on Wii? Reflex is a great game, but if you'd rather aim with a thumbstick than with a Wiimote this game will be as any Wii FPS: not so good. But if you like Wii aiming, then this game is very awesome; not as good as CoD on other consoles because of the graphics, but if you're a gameplay-over-graphics guy you'll be fine. It also has lots of repay value, since it has online and you'll always have something to do.
New Mario Bros: What gan I say? If you liked it on DS, you'll love it on Wii. Sure, it isn't portable, but the gorgeous graphics and the tight control (better than DS in my opinion) make up for it. Also I've heard it's harder (I'm only in world 4); if that's good or bad it depends on you. Also, it's ok if your friends don't like Mario games: it's still a blast to play alone.
So personally I'd get CoD or Mario, depending if you prefer shooters or platformers. I myself play Mario more, but it's because I'm not that fond of shooters anyway.