I think it would be extremely hard to say who was the best in history. I mean what Waffen SS ? Not all were the best, some without a doubt were some of the finest fighting units in history, but then some had low training, and horrid tactics, but the best? I don't know, would not say they were either.
US Marines are, where, and still are an excellent fighting army. The war in the Pacific showed their ability to fight, not to mention Korea, Vietnam and modern wars. Would not say they are the best in history but they are a highly trained, and hard fighters. For Bad Tolz to say they do not belong the list is rather a silly statement, or lack of knowledge.
I also think parts of the Red Army are never given due credit for their fighting ability. Chikov's 62nd Army at Stalingrad would be a good example.
Roman's, is hard to say considering their enemies.
The best is most likely a personal choice. I pick Marines simply because it is a branch of my countries military, and they have done an excellent job for this country through out their long history. Plus I am a rabid patriot may have something to do with it also.
Absolute best is hard to say..... Maybe the Mongols just because of their success, hell they never were defeated, I think they just got bored