LooftWaffles
Smash Journeyman
You are unlikely to defeat high caliber MKs if you think this. Not knowing where the openings are, where the reads are to be made, how to deal with Mach Tornado (knowing when it's a threat, how to punish, how to deal with it's pressure, and when you must be defensive because of it) all of that for shuttle loop as well, kill percents, viable combos, range discrepancies between moves, and a whole host of knowledge, then you're unlikely to have a chance at beating high level MK.
Marth's strategies of "zone, hit, and punish" lose to MK BADLY because of his range, speed, shuttle loop, and Mach Tornado. Also, you need you use specific techniques to juggle and edgeguard this character because of his ability to fly and recover in different ways. The reason most Marth's do not edgeguard MK is because they don't know how.
MK is a very intense MU to learn, and it's very important.
Diddy is HARDER to learn though, IMO.
Way to misread my post
First thing I established was that MK was the most important MU. I dont think anyone competent could argue otherwise. Other MUs may be considered harder by individuals, but do not pose nearly the threat MK does in the metagame.
As for learning the MK matchup, its pretty simple in my view. Marth has to learn everything he does in any other mu- safe zones, unsafe zones, kill moves, recovery (both ways) and proceed to go to business with his sword. This isnt the case in the Diddy mu.
The Diddy mu brings Lunch. Peanuts and Naners. No matter how hard Marth tries, he cant spawn items. Neither can MK ( He wanted to, but chose shuttle loop instead). Marth not only has to deal with how Diddy uses the items against him, but needs to LEARN FOREIGN OPTIONS.
Things Marth cant normally do:
Glide Toss
This alone is HUGE. Takes Marth spacing to a whole new level. Pretty self explanitory, and kill setups are always nice.
Other things bananas do:
Cause techchases. Both ways. Learn your options with no, 1, and different 2 banana spacings.Both ways, because you know the Diddy has his end covered.
Punish OoS. Seriously, think about this for a minute. Diddy's throw OoS is such a vital tool in this mu. Now think about what Marth can do shielding with a banana.
Dolphin Slash
Shield drop DB
Aerial counter (faster)
Normal Toss
Glide Toss -> Shenanigans
I definitely think learning the Diddy mu is more important than learning the MK mu, its just that youre more likely to fight an MK