Best combat bolt action?
Lee-Enfield, without a doubt. 38 aimed rounds in the Mad Minute drill. The Mausers or M1903s are not as good, and the French were stuck as early adopters of smokeless with a much less than ideal cartridge. The various 6.5mm rifles were good calibers, but generally lesser ergonomics than the Lee-Enfield. I'd just about give my left nut for a Long Lee, the Magazine Lee-Metford.
That said, the .303 is a bit overpowered for most infantry combat post 1914, once the threat of having to shoot horses passed. A Lee-Enfield chambered in 6.5 Arisaka is just about ideal. Note that the Brits basically designed the same thing as the 6.5 Arisaka after WW2 as the .280 British. 140gr bullet at ~2400fps.
Best combat semi-auto (Garand, SVT-40, G3, FAL, AR-10, FG-42, etc)? Tougher question. I'd probably have to say
FAL, at least in terms of number of adopters. The M-14 is pure fail here, even the SVT-40 beats the M-14. Runner up is the
SKS. Not a full power rifle cartridge, but still a very solid and accurate rifle. Still used as the ceremonial rifle in all the former-COMBLOC nations.
Best assault rifle?
Early M16s, once they got the chrome lined chamber and barrel they were always supposed to have. Almost as light as the M1 Carbine, and ammunition was much lighter (~2/3 the weight of M1 Carbine ammo per round). Later M16s and M4s have mostly gotten heavier and heavier
before adding all the extras necessary for modern combat like lasers and optics, it's a major pet peeve of mine. Runner up is the
FAMAS.
Le Clarion has surprisingly good ergonomics and is quite good as a short and handy rifle for mechanized troops. Needed a third production run in about the 2000s, though, to give it picatinny rails and STANAG M-16 magazines. (These days, the runner up is the Croatian VHS-2/Springfield Hellion, since it has Pic rails and STANAG mags). Epic fail award is the SA80, though I still want one of the L86s with the long barrel.