: Why does this progression sound so good? What is the first chord that makes the CMaj9 sound so good? And what is that progression called?

TL;DR : there's an actual answer to the "why" question in the end. The recipe is: move (absolute) voices somewhere and make the listener re-think the (relative) harmony. That's basically all there is to jazz harmony. Don't settle for the most obvious choice, put the listener's imagination to work.

"Why does X sound good" is the wrong thing to ask. It's one of those common idiomatic things to say, and cannot be taken literally. Just like "Where did these chords come from". You mean to ask "please help me understand what's happening here" - and "understanding" means things like this:

I want to see this in terms of other patterns I'm already familiar with
I want to be able to utilize the structural patterns in my own music in a controlled way
I want to know what I can do when I encounter this pattern (this is what people want when they ask "what scales should I shred if there's such and such chord")
I want to be able to identify this pattern in other pieces of music in the future
I want to have a name for the pattern (or its components), to help me categorize, handle and communicate things
If the pattern can be seen as a combination of simpler more atomic patterns, I want to know those.
How can the pattern be categorized - what other options are there to perform similar functions?

If you can answer those questions, then you can really say you understand it!

It's not possible to learn all this in a "one question, one answer" way, as if all of the listed perspectives could be pushed into your head as pure information. It's a learning process, like learning to taste different food ingredients and how they interact and how to utilize them in your cooking. The answers we give can only provide source material and suggestions for your personal and individual learning process.

In my experience, the way to obtain the "understanding" i.e. handling ability, is to interact with the notes and chords, and see how the interactions affect your feelings and possibilities. You try to simplify and modify the chords, and play different notes on them. If you add or remove notes, does it change the essence of the harmony? What scales can you play on it while retaining the same harmonic feeling? Can you replicate the same harmony in terms of simpler chords? Where do you feel the tonic could be at different stages of the progression? Where does each note "want" to resolve? Can you play it in different keys? Can you apply the same pattern in an existing song you're familiar with?

I don't know if this helps in any way, but what do you say about this - does it have the same essential characteristics of as the original chord progression? Is something different? I interpreted the first chord as E♭m6 or Cm7-5

Then again, wouldn't this sound even better? Now it resolves to a B♭ major chord and the leading melody line is kept the same.

Is that too obvious? The original had a bit of a surprise in it, and now the surprise was removed.

But I think just about any voice movement sounds good. This sounds good too:

The movement makes my ear accept almost anything, but there's an additional interest boost, because each new chord makes me re-think the harmonic context. Sort of like what they do with the chromatic modulation in Eurovision Song Contest entries. Move and rethink.

This sounds good too. Now we keep some voices in place and move others. Each new combinations sounds interesting.

You could try to "analyze" the chords and see functions, but IMO the magic comes from the same recipe: move voices and make the listener re-think.

The one with the Cm7-5 - B♭maj9 progression did move, and re-think a little bit, but not much. The original Cm7-5 - Cmaj9 makes you re-think more.

The same melody and initial chord could go to any number of places. Most of these end up somewhere around Gm or B♭, but not all.

So why does this sound good, how about this why does it sound good. Why does anything sound good. Move and re-think.

---

So with support from @John Belzaguy I think we have come to a possible answer:

The chord in question (C, F, G♭, B♭, C, E♭ <--> F) resolving to a CMaj9 can be seen as a A♭13 or its tritone substitution D7alt. Other options could include a Cm7♭5 (with an added 4th = F)

Respectively, the movements would be IV7 --> I , II7alt --> I, or Im7♭5 --> I

I guess that although this does not resolve a tritone to the tonic, the voicing in all mentioned cases have very smooth movements of half-tones into the resolving chord, so it still makes for a satisfactory sound.

---