In terms of learning the most? I'd say
1. M2K
2. PPU
3. Zoso
4. Borp's pocket marth
5. PPMD
6. The Moon
7. Swiftbass
8. Taj
9. Tai
10. Ken
M2K's marth is pretty much a giant bag of neat tricks. Lots of little things you can take away easily. You can't actually model you're entire game on neat tricks unless you're M2K, but in terms of "implementable things/minute", M2K is probably the best. M2K is probably, overall, the most copied played, and it's for a reason. PPU has a lot of that in his punish and combo game as well.
I put Zoso and Borp higher than PPMD for a number of reasons. PPMD's biggest strength is his spacing in neutral, but what a lot of people don't seem to notice/mention is that his neutral game is largely not really repeatable for most players. When he does his dash dance, there's a lot of small, subtle reads he makes about how the other player is going to move, and those are things you can't really learn by watching. How to read another player in neutral is probably the hardest thing to learn by watching. It's just something you've got to DO for yourself a crapton to understand. A good analogy would be to say that PP's dash dance into d tilt is basically a much, much safer version of Azen's neutral f smash.
Yes, Azen makes random forward smashes in neutral work.
No, you probably cannot make random forward smashes in neutral work.
Zoso, by contrast, is probably the best player to watch if you're looking to realistically "model" your game after someone. He's probably the best marth that relies as purely on fundamentals as he does. It's just decision making, basic movement tech, spacing, some basic combos, and then decision making some more.
Borp is a similar, if extreme, story. There's not much footage of his marth out there, but what is there is pretty much what you'd expect. Pure spacing, timing, and decision making. A few hard reads, but nothing insane like Ken. Only issue is there's lots of times where he'll pick a not safe option that he wouldn't have had to if he used tech skill.
The Moon's earlier marth stuff was probably a lot more helpful to watch. Lots of baits, combined with stellar movement. Now he does a lot more reads and pure callouts in neutral that are just harder to learn from.
Swiftbass, Taj, and Tai are all also mostly fundamentals based, but don't have the same spacing and decision making that Zoso and Borp have to make them as educational to watch. Ken's marth is good to pick up on a few weird things Ken knows, but it's mostly a style that can't really be replicated.