Which is the best daily driver upgrade for celi gts? Post your opinion. Why?
Do you mean the Celica GTS will function primarily as a daily driver and see some race time? Or that it will only be a daily driver and that you just want more power on the street?
IMO, the Celica GTS with standard I/E and PowerFC is plenty of power as a daily driver, especially since the PFC adds so much low end torque. If you put a turbo in your Celica, you can expect your fuel economy to go down so if the car were ONLY a daily driver I would not recommend it. I would only turbo if I planned on racing my GTS on a constant basis. S/C is not worth it, unless you want to road race.
If I were to race only once in a while, the answer would be:
Relatively the most cost-effective for occasional racers. Simple, easy to install, or uninstall if you want to pass a smog inspection. Safe if done right. Plenty of power.