Best grinder for USSR probably the Su-25K, Su-11, Mig-23ML In order of grinding efficiency. Su-25K is probably the easiest plane to do well in. Su-11 has broken flight performance for it's BR. Mig-23 is good for its BR but it is 11.3 so you have to know what you are doing near top tier or you will get slammed.
For the US the A-10, and the F-5C, A-10 is easier to grind in overall but the F-5C has a higher potential of grinding more RP per match if you can consistently do well in it.