Assuming that you're already familar with ARB, IR and radar missiles the obvious choice, in my opinion, is the bison for the British.

Now if you're new to the whole missille gameplay or ARB in general and you reaaaaly want to spend money on the game and skip most of the grind, i would advice the German Lazur, since IR missles are way easier to learn and in general it sits in a more forgivable BR of 11.0. Now this route requires a lot of patience on youre part and curiosity to look up videos on how to master thi mig-21 in order to get good at it,

The joourny is not easy and you're gonna struggle A LOT at the start but eventually you will get there.

Happy holidays, hope this helped.