Ask that question to 10 different people and you'll get 10 different answers.
This is what I have noticed to be popular around the people I race with in 21.5 blinky.
Motors. (in no set order)
Esc. (in no set order)
Tekin RS, RS Pro
Hobbywing Xtreme Stock
Hobbywing V2 60A SD
Hobbywing 1/2 Cell
There's probably a few more that I have missed.
Also depends on your budget, Last year I bought a Tekin combo of RS esc, hotwire & 21.5 motor for about $300AU. A short time ago before the AU$ took a nose dive I picked up a Hobbywing Combo of 60A Xtreme Stock esc (not the Justock), LCD programming box & 21.5 motor for about $160AU.
I'm now having more success with my Hobbywing combo then I did with my Tekin, sure the Hobbywing motor has no timing on it but with it's torque I'm running a roll out of between 71-72 mm
and the Hobbywing esc feels more "smooth" then my Tekin does.
With all the choices out there you have a lot of combinations that may or may not work for you, I got lucky with mine and am happy with the combination of performance and value for money.