First, it seems to depend on you needs.
The TRD is a decent compromise one (each intake is a compromise of several factors), at a relatively decent price for what you get (it still ain't cheap), and doesn't void warranty.
There are several other popular ones too. Some seem better for sound, while others give more power increase.
-I am fairly new to Scions, but have been playing with Miatas for over a decade. I have picked up a few things there that seem relevant to xBs. I am looking (casually, as I can't afford/justify one at the moment), so am learning about the products.
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