Can you elaborate a bit? Those are the diatonic (ie using only the notes of the scale) seventh chords on ii-v-i in a major key. In a minor key, you ususally get half-diminished, dom 7 flat 9, minor 7 (or minor with maj 7). Or am I misunderstanding the question?
If you just say 2 5 1 , then it will generally mean a diatonic 2 5 1, exactly as Bigmartin says.
However if you also include chromatic chords, then the II chord could be a dominant type chord. This is called a secondary dominant and is often the dominant of a dominant:
D7 G7 C major 7.
Normally when people talk about a 2 5 1, the V chord is always a dominant in major or minor (perfect cadence).
An exception might be if the sequence is modal, in an Aeolian mode a diatonic 2 5 1 would be Bm7b5, Em7, Am7. But if that was the case, people would say so, this is not what is typically meant by the term 251.
ii-V7-I is by far the most common version in a major key. They're diatonic, also perhaps more importantly they follow a subdominant to dominant to tonic pattern (we've been using this cadence for like 500 years or more. It still works). Typically in a minor key they won't be strictly diatonic (as Pete alluded to), the V will be dominant.
Probably the most common variation is using the dominant 2 chord instead of minor. Much more common in pre-war jazz than in bebop and later styles.