To hear what's going on in imitative contrapuntal music, you need to follow multiple melodic lines simultaneously and match what you've heard in the past to what you're hearing in the present. How difficult this is depends on many factors, including ...
Following is a collection of animated graphical scores of music with canons with various degrees of complexity.
The simplest canons have two voices and no added complexity.
In these, the main factor contributing to how difficult they are to follow is the length of time between the first and second appearance of the melodic material.
This piece has three voices and a long time interval between them:
There are four versions of the graphical score for this one:
This canon is unusual in that it includes a variety of playing techniques: arco, pizzicato, jeté, portamento (glissando), and percussion (tapping the instrument):
From The Musical Offering (BWV 1079); this is a puzzle canon that can be realized in more than one way:
This is the only six-voice canon I've ever come across; I've done several versions of the animation:
It's probably best to first follow the first part (in white, corresponding to Jesus) all the way through a few times, just to get familiar with the melody and the transitions across the joints:
This one pushes the definition of "canon" to an extreme: it's a percussion piece, so there are no melodies, and the time interval is so short that it's hard to say whether it's a canon or merely an echo:
Two canons running in parallel:
Canons in inversion
In these, the imitation is inverted (melodic intervals are preserved, but up/down are reversed):
Canons in augmentation and diminution
Conlon Nancarrow's canons are quite different from Bach's:
Canons in augmentation and inversion
In these, the imitation is both augmented (played half as fast) and inverted:
Retrograde canon (cancrizans)
In which the imitation is time-reversed:
Canon in retrograde and inversion
In which the subject is both inverted and time-reversed: