T61 turbo, but there's a lot you're going to have to do before id suggest running a larger turbo. Your stock ECU wont boost more than 11 PSI, so starting with a lexus AFM, bigger injectors, and then lastly a new turbo. Other questions to ask yourself:
-Do you have the proper gauges in your car?
air:fuel and boost gauges are both IMO mandatory.
-Do you have a boost controller?
-How much power are you realistically trying to make?
-Is the upgrade to the T61 necessary for said goals?
Just some stuff to think about before dumping your wallet. My turbo is upgraded 60-1 and it makes plenty power to enjoy myself, turn heads, AND make power pre 6k RPM haha.