There ain't no best, it all depends on your application your mods.
If you want to have an easy installation, I'd go with the "plate style" from Nitrous Express (~500 bucks)
If you want to have a professional, more fancy style system, I'd go with "injector spacer system" from NOS. (~1300 bucks)
Keep in mind, that with a stock motor you don't want to go bigger than a 100 HP shot.
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