Against the circle of fifths for practice drills
It's a very common recommendation to drill harmonic or melodic material in every key by running it through the circle of fifths: do it in C, then in F, then in Bb, and so on. If it's in minor, then you start with A minor, then D minor, etc. (This post is aimed at people who are already familiar with this and know the importance of getting comfortable with the falling 5ths sequence.)
There's a practical problem with drilling this sequence on a regular basis though.
As you move from C to F to Bb, you're gently easing into the flat keys. But once you've crossed the halfway point of the circle, you've now done the opposite for the sharp keys by starting with the most sharps.
If you always have enough time to cover the entire circle and always get to the end, this isn't a problem. But personally, I find that I sometimes don't make it to the end for various reasons, e.g. something came up and I had to cut my practice short, or I got carried away drilling the first 5 or 6 keys and have used up all my practice time.
When this happens, it means that the key that consistently gets the least attention is G, and then D in second-last place. But these are extremely common keys!
So here's one alternative: prioritise the most common keys first.
If we assume the most common keys are the ones with the fewest sharps or flats in the key signature, your sequence might look like this:
0 1# 1b 2# 2b 3# 3b 4# 4b 5# 5b 6#/6b (7# 7b)
C, G, F, D, Bb, A, Eb, E, Ab, B, Db, F#/Gb* (C#, Cb)
* Since F# and Gb are enharmonic to each other, it's common to just treat this as a single key (and skip C# and Cb entirely since they're enharmonic to Db and B).
Another minor benefit of this approach is that unlike the Co5, where each key is maximally similar to the last, the amount of similarity between keys here changes with each step, with maximal dissimilarity in the middle (Bb → A, Eb → E and Ab → B, all with tonics a semitone apart) and a tritone jump when looping back to the start (F#/Gb → C).
Because you start at C and end at F#/Gb, this means you're travelling down the Co5 from the top rather than clockwise. I like to visualise the circle as a sphere that starts full of water and is gradually drained throughout the exercise :)
You may find that other custom sequences work better for what you want to practise. For example, if certain keys with lots of sharps or flats are common on your instrument, it makes sense to prioritise those. Or if you're already comfortable with the common keys and too rusty on the less common ones, you could flip it around and do F#/Gb first.
Happy practising.