if you already have a motor then I would say use what you have as long as it is the right scale and you will not burn it up (check running temps often during setup).
from what I can tell most of the pros are going with Tekin R8 system and I hear it is a great set up but it costs like $350. I got a clearance castle creations motor and I bought a ESC from hobbyking for 70 bucks. so after I get the conversion kit from Losi I will be into the conversion for about $210 I think without any batteries yet. I am trying to do it on the cheap side, if I went with the Tekin R8 I would be into it for $420 without any batteries and I think that is to much since I am not sure if I will race or not, but for now I am aiming to just bash.
I dont think there is a best combo persay but more on how you set it up, if you just slap in the most expensive setup it could still suck until you tweak it. also from my research everyone is recomending lower Kv motor for more torque and you can just gear up for speed. they say it is safer that way because if you run a higher Kv and you have a hi grip track you will heat your motor up quicker I guess and risk ruining your expensive brushless motor.
I think that why Losi sell these combos with these Kv's
1/8 Brushless Motor and ESC Combos
LOSB9556 – 1/8 Xcelorin 1300Kv Brushless Combo
LOSB9557 – 1/8 Xcelorin 1700Kv Brushless Combo
LOSB9558 – 1/8 Xcelorin 2100Kv Brushless Combo
Others say lower Kv is better for Truggys and Monster trucks because they weight more and have more traction so you need more torque and the higher Kv motors are for the lighter buggies. just what I have found from my own research, I may be right or wrong lol. I recommend doing research yourself and see if what you find matches my info.