The chords which occur most frequently in any key will be those whose notes are taken from the key’s major scale.
E.g., in the key of C the most likely chords to appear will be those which contain all natural notes (i.e., notes from the C scale). Here are 25 such chords, which can be said to "belong" to the key of C.
A thorough knowledge of these chords will help you decide the key of a piece (and therefore the location of improvising patterns) where no key signature is given. For example:
This progression is in the key of C, using the chords:
Another way to decide the key of a progression is to look at the first and last chords. These will often be the same, and also have the same name as the key. E.g., for a progression starting and finishing on F, the key is most likely to be F.