the game was different then, so it's hard to say, but in most things the "technology of training" advances so much that, were the older teams to play today, they wouldn't look nearly as good as they did in their time. so we have to talk achievements, and i'm not quite ready to say this spanish team is the best ever, though i think they are in the conversation, and may be #2 at this point.
1958-62 brazil is still the only team to win 2 world cups in a row. and 1958 brazil were utterly dominant, in a way spain has not been. that team is still #1 for me.
that's the only team we can even talk about, at this point, still being "better" than this spanish team. well, if you extend the timeline a bit so that it includes multiple generations of players, you could say 1994-2007 brazil team that won 2 world cups, got to the final of 1 more and won 4 copa americas...but that's comparing apples to oranges. it is the most sustained period of success a national side has ever had, though.
you could also make a case for 1972-76 west germany team that won 1 european championship, then the world cup, and then lost in the final of the second european championship on penalties. but i don't see the rationale for ranking them above today's spain--that german team won its world cup at home, which is always easier than doing it somewhere far away, and didn't win that second european championship.
the dutch team of the 1970s was awesome, but never won anything.
if this spanish side win the world cup in brazil, it is definitely the best of all-time.