I have handled and fired quite a few semi/full-auto rifles, including those mentioned by telewinz plus the SVT 38, G.41W, G.41M, G/K.43, Pedersen, Johnson, MP.44, the Hakim, the Ljungmann (the original of the Hakim), Rasheed, the AK-47, Czech 52, SKS, the AR-15, AR-18 and 180, the original AR-10, etc. Also a bunch of light/medium MGs, Lewis, BAR, MG.34, MG.42, M1919A4 and A6, FAL in LMG role, M14 in LMG role, etc.
As a full power military semi-auto, the M1 is hard to beat, and I think the M14 is its only close competitor. The FAL is better in full auto because of its weight and the more straight line stock (the M14 is about worthless in FA), but it is heavy and awkward, with a long receiver that takes too much from barrel length. The fact that it was adopted by many countries is as much a fact of availability as of its actual quality. (If we consider numbers issued as equating to quality, the AK-47 is the best rifle in the world, bar none.)
The M1 was hardly perfect. One of the problems was that GI's finding themselves in a lull in the battle with a partly empty magazine, tended to fire off the rest of the clip so they could reload with a full clip. But the M1 was a far better rifle than any of its contemporaries. Were there better semi-auto rifles at the time of its adoption (1937)? I can only say, name one. The only other one that was in service that early was the Russian Simonov AVS 36; I have never even seen one outside a museum, but the Russians did not continue it in service very long so it must not have been a world beater.
The AG42B was not adopted until 1942, and was not especially liked even by the Swedes; they never attempted to issue them to their whole army. It was adopted by Egypt for political reasons (Sweden was not in either "bloc"), not because they especially liked it. The German rifles were decidedly inferior to the M1, in part because of too fast development and wartime production, but also because of (IMO) inferior design.
The only rifle that could have been up there was the Johnson, and it was also not as good as the M1, though it was close. But the Johnson is awkward with its magazine bulge, and many of the smaller parts were not well designed for hard service. It did, however, solve one problem with the M1 - it could be loaded with standard M1903 clips and could be "topped off". The ability of the army to supply the "special" M1 clips was a large concern at the time; it actually proved no real problem at all.
As to LMGs in the rifle role, I can only say that anyone who would prefer the Lewis or the BAR over an M1 has never carried either one or has a lot more muscle than I ever had. Those guns are not bad in the squad auto role but not as standard infantry rifles.
The medium power selective fire rifles (AK, AR-15/M16, etc.) are in a different category, but they did not exist in any numbers until late in WWII.
Jim