Oh wow, this thread is not what I thought it was at first. I'm glad I found this, good idea, Verto. ^^
Honestly, this is an easy choice. I would pick Twilight Princess without a second thought. I've got several reasons for this, starting with the difficulty.
For one thing, I wouldn't recommend a 2D Zelda game like ALttP or LA, only because those are usually harder to an inexperienced gamer than a 3D title. Twilight Princess is often thought of as the easiest Zelda game, so why not start there? The controls are very fluid, at least in the GameCube version. This would get a gamer new to the Zelda world aquainted with the controls, because, for example, the blocking, is fool-proof. Unlike OoT, enemies' attacks in TP hardly ever hurt you with your shield up. Also, you only have to press one button to defend and target, also unlike OoT, and Wind Waker as well. As far as difficulty, I would pick either TP or WW to give to a newbie, probably the former.
Another thing is the graphics. Someone who is used to modern graphics might be a little turned off by OoT's graphics (and slightly shaky controls). Again, in this area, I might recommend TP or WW, but possbily PH or ST, even MC, as well.
I think I would start someone off with a console Zelda game, just because the difficulty level isn't too frustrating, and they're easier to master. In this regard, WW or TP would be perfect. If the person (whoever they may be), preferred a handheld Zelda game, I'd probably give them PH or ST.
Overall, I think Twilight Princess is the perfect game to start someone out on, and let them get the feel of the series. TP was the first Zelda game I ever beat, and now I'm hooked on Zelda. I'm pretty sure anyone else would feel the same way.