Battle Rifle -- M1 I would trade the ping over feeding stripper clips in a bolt gun any day. The Enfield would be second on the list. Talk about stripper clips though, the Enfield clips are a royal pain to use.
Pistol -- .45 1911. More stopping power with the same relative ammo capacity of the 9mm of the day (exception Browning Hi Point) Really a great, reliable pistol. Browning HP second.
SMG -- I'll go with the PPSh 41. Buzz saw up close, but with more velocity/penetrating power for shooting through cover and maybe more midrange accuracy than a .45 smg. More battlefield useful, perhaps. The real winner would have been a more widely used select fire M2 Carbine which is the perfect WW2 weapon for the traditional SMG role -- room clearing to 200 yard ranges.
LMG -- Mg42/Bren Gun depending on tactical doctrine.
Grenade -- US Pineapple over Potato Masher, though don't know enough about the British Mills Bomb or the Russian grenades.
Sniper Rifle -- MN 91/30 PU. Why, because it is the only one I could afford to buy and still shoot
Also, it may not be the absolute best, but it was widely deployed as part of a "squad marksman" concept of soviet battle doctrine. Good, solid, reliable scope and rifle combo. On a side note, the turned down bolt vs the standard straight bolt eliminates the only major flaw of the 91/30, IMO. Easy as butter to work with that change in leverage.
Personal ATW -- Panzerfaust.
Weapons that changed the post war world -- MG42, Bazooka/Panzerfaust and StG 44.