I didn't see anyone answering this question for you.
The RS is a NA 2.5. It's the slowest Impreza. From 2002-2005 the WRX has a 2.0 turbo. From 2006-present the WRX has a 2.5 turbo. The TR is just a WRX without some of the creature comforts. If I were in the market for a WRX, I'd go with an 06+ model. The turbo 2.5 turns into a torque monster with a little tuning.
Of course, the STi is superior to all of the WRXs. It's a much better starting point if you're looking to mod. When all is said and done, my top choice between any Impreza or Lancer would be a 2005 STI (obviously I'm biased in the EVO/STI debate

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