tl;dr There is lots of variety for all senses across people, and lots of variety between senses for the same person. Histograms of responses (what most people probably want).

The motivation behind this was that I am 0 on everything but sound where I'd be around a 6. I only realized this wasn't normally and people weren't talking metaphorically about "seeing things" in their mind last summer (and I'm in my 20s).

The inability to imagine senses (typically visuals/sight is the one of interest) is called Aphantasia. It has very little research on it to date, basically just demographics/distributions like this. If you're curious to read more, there's this blog post and this youtube video for some personal takes. You can also check out /r/Aphantasia, /r/hyperphantasia, and tangentally related is /r/Synesthesia.

Alright now some numbers. First up is clarity of imaginations (1-10 scale):

Sight: Avg - 7.45, Median - 8, StdDev - 2.36
Sound: Avg - 7.78, Median - 9, StdDev - 2.39
Taste: Avg - 5.26, Median - 6, StdDev - 2.82
Smell: Avg - 4.62, Median - 5, StdDev - 3.13
Touch: Avg - 5.72, Median - 6, StdDev - 3.04

Sound and sight are the clear front-runners for clarity, and are more consistently high.

Next up is the likelihood of involuntarily imagining a sense throughout the day (1-5 scale).

Sight: Avg - 3.21, Median - 3,   StdDev - 1.28
Sound: Avg - 4.24, Median - 5,   StdDev - 1.01
Taste: Avg - 1.94, Median - 2,   StdDev - 0.92
Smell: Avg - 1.85, Median - 2,   StdDev - 0.93
Touch: Avg - 1.77, Median - 1.5, StdDev - 0.94

Again here sight and even more so sound take the lead, I imagine due to imagining songs and conversations (total conjecture on my part here).

Now some random stats I decided to check out. First correlations between clarity of a sense and likelihood of involuntarily imagining that sense.

Sight: 0.62
Sound: 0.41
Taste: 0.49
Smell: 0.50
Touch: 0.40

The last thing I looked at was the variance of clarity of different senses for the same person. I took standard deviations of all the senses for each person, as well as the largest difference (their highest rated minus lowest rated).

StdDev: Avg - 2.07, Median - 2.05
MaxDif: Avg - 5.44, Median - 6

Thanks for everyone who responded! Hope everyone else finds this variation as interesting as I do.

Raw data