1. MK
2. Wolf
3. Sheik
4. Mario
5. Pit
From my experience, I found Mario the easiest to pick up on, probably because I played with Mario in Melee, but I always thought of him as the "standard" character; just the basics. After that, I would have to say Pit was easy to pick up on, I picked up on him within 30 minutes of playing the game, as he's pretty easy to use, he's not too light, he can hit from a distance with arrows, he has a good shield, and he flies so it's easier to save him. Those, to me, seem like the basic things someone new to him would use repeatedly, and being that it seems relatively effective, one would keep using him and get better with him in a short period of time.
I picked up Meta Knight shortly after discovering SWF to see what everyone's love affair with him was about. I picked up on him pretty quickly, but I simply figured I
felt as if I had picked up on him quickly, and that there were much deeper aspects that I had yet to understand. I didn't know he was generally accepted to be easy to pick up on.
I hate Wolf. He's a heavier Fox/Falco, but his move set just feels clunky and a little off, so I've toyed with him and then immediately put him down a number of times, increasing the amount of time that it takes for me to pick up on him, that is, if I ever do. He's simply, for me, a turn-off, so trying to get good with him is something that I may never do. This could be only from my perspective, but because of almost everything about him, I think he would be the hardest on this list to pick up on, and Fox/Falco both would be a little easier. Wolf's just strange.
Why does his forward-B angle up? That came in handy, like, once; to get to a platform quickly (on Pokemon Stadium) and grab a fan.
So I would say:
1 MK
2 Mario
3 Pit (really only because aiming his arrows effectively takes some time)
4 Shiek
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5 Wolf