FrogReaver
The most respectful and polite poster ever
Hideous laughter as an example is stronger action denial than stunning strike on a per use basis. So Is command (flee).Stunning Strike is likely the defining aspect of the base monk class. The monk features support a skirmisher style - able to get to vulnerable targets. It's not a full striker/DPR as mechanically supported. When you pick targets, like enemy casters, with low CON, Monks become action denial specialists. While CON is one of the worst saves to go against, between choice of target and the unique ability to force multiple save-or-suck saves per round is quite powerful. That multiple-save-per-round is also great for doing things like draining Legendary saves from a big foe.
So you can't look at the value of a single Stunning Strike, but rather in being able to repeat it in the same round, like casting multiple spells, and how it makes all of the rest of your attacks that round and the next round with advantage. (Which also makes it easier to reapply.)
Each Stunning Strike is less powerful than a multi-target action denial like Fear or Hypnotic pattern, but is more powerful than a 1st level spell like Tasha's that doesn't contain the "keep repeating for each hit" and avoids the "attacks give another save" vulnerability. So I'd place it around a 2nd level spell in value. Which makes it quite worthwhile compared to other Ki costs.
(As a side note, the Four Elements monk is generally considered weak, and when you take that out of the equation Ki seems to be more valuable than 1 spell level per Ki.)
Expeditious retreat, another first level spell can mimic most of the monks other uses of ki.
Shield spell is comparable enough to the bonus action dodge use of ki for the monk, except typically better because it’s a reaction.
Monks ki abilities are mostly equivalent or a little worse than first level spells.