My favorite Mod so far is the Steeda IRS sub frame braces. That has been the most notable change in handling.
My least favorite would be a toss up between the jlt catch can or sound tube delete. I like what the mods do but since I don't look under the hood regularly i don't really notice them.
Favorite mod: getting rid of my base ecoboost for a premium preformance pack ecoboost
Worst mod: boomba raciong bov (think it was on the car for a couple hours, took it off and sold it)