General and best ways to cutoff frequencies?

[trogluddite]

If we take the discussion to offline processing, that is not in real time, do the same restrictions apply?

Essentially yes, the fundamental limitations of any given technique are the same, though of course, without latency to worry about, you'd have more options for what compromises you make. You could also use other, less efficient, techniques in some circumstances (e.g. auto-correlation for pitch detection), or maybe do multiple passes through the samples (e.g. a frequency analysis pass and a transient detection pass.) The basic problems of trading frequency precision versus time precision, or precision versus leakage, still don't go away, unfortunately.