Her dash attack is great, especially against fast fallers, for all the reasons that have been said, but if i were pressed on her best over all move IMO its the nair. As was said earlier, it has great priority, it also has decent knock back (very good by nair standards), little lag, you have the option of spamming 3-4 (maybe more, dunno) nairs while floating, followed up by fc-> move (dsmash), or just short hop nair.
The dash attack can be a great set up for combos, especially at low-mid damages, but at higher damages, there is too much knockback to do these combos or an aerial attack. The nair, however, has great knock back and ko potential at higher damages, and it can be a great way to molest opponents when they get knocked off stage.
Im definetly not knocking the dash attack, but if I were going to pick one overall move, I think that her nair is it.
Note 1.: Im not a pro peach player, but she is quickly becoming my main.
Note 2. I wrote that before i even thought of the turnips (high, oops), and maybe its the turnips...They are pretty unbelievably annoying if u use them correctly, but....im still sticking with nair, i think.
Edit: Hmm, i read this after I posted it, not sure if its correct or not.