Whenever a chord progression is being played, you are always trying to resolve back to the tonic. Not every chord resolves directly to the tonic, but they will resolve indirectly. Let me show what chords resolve directly to the tonic.
Chords that Resolve Directly to the TonicThe chord that resolves to the tonic the best is formed from the 5th (V) degree of the scale. The reason that this chord is the best is because it contains the leading note (7th degree). In the Key of C, B is the leading note because it is the note before C. Look at the chart below.
C Major Scale
C Major Notes
G Major Notes (V chord)
C Major Notes
The diminished 7th chord also resolves nicely to the tonic because it is formed from the leading note (B). In the key of C, the diminished 7 is B diminished 7 which contains the notes B, D, F, Ab. This resolves nicely to the tonic because the B resolves to the C (and Bb if you are playing a C7), the F resolves to the E, and the Ab resolves to the G. Look at the chart below.
B Diminished 7 Notes
Chords that Resolve Directly to the VThe II chord resolves directly to the V chord (and therefore indirectly to the tonic) because the II chord is the 5th degree from the V. In other words it is the dominant of the dominant. If you took the V chord (G) and wrote out its major scale the 5th degree of that scale (G major scale) is the same as the II chord (in this case it is D). This is where we get the II-V-I Jazz Progression.
The IV chord also resolves nicely to the V chord but not as well as the II chord. This is where we get the chords of the 12 bar blues (I-IV-V).