Definitely either Marth or Sheik if you're just trying to get used to the game's engine.
A. They're pretty easy to teach with, at least at the very basic level. Everyone knows how to play both characters to an extensive degree, plus each character isn't reliant on a gimmick.
B. They're combos are pretty easy, at least easy in the sense of "oh, I can string these moves together into a combo." The constant use of these combos enables the user to learn the very basics of stringing moves together with each character. More than anything, after teaching the first few combos, they'll probably be able to figure it out on their own if they want to (as opposed to, say, Lucas).
C. Each character has very good strengths but at the same time noticeable flaws. They aren't immediately apparent, so they're less likely to john about them if they're at a low level (those this depends on the player). Plus, there will always be areas where the player feels like they can improve on, as opposed to being stuck on a certain glaring flaw and being frustrated by its limitations.
D. They have good grabs, plus they each have a combination of both combo grabs and tech chasing grabs. It's good for practicing. Marth's grab range is also pretty generous, so many players will feel comfortable with that range.
E. If they ever want to get to that level, both have good wavedashes, especially Marth. Marth's one is especially intuitive since Marth's game is already about movement and spacing rather than aerial superiority.
F. Speaking of spacing, Marth has a sword. His sword practically teaches spacing by itself.
G. Sheik has a projectile. They can learn how to abuse them, but not be too punishable at the same time.
H. They're both fast and have lots of mobility, but not too fast in the air or ground to the point where it's hard to keep track of (ex. Fox).
I. They can both edgeguard well. It helps teach the basics.
One of the bad things about them is that they don't really have very good Oos options, so you can't exactly teach that well. At the very least, you can teach them Marth's up-b out of shield and shield grab.
People are also saying Mario a lot. I disagree, since Mario's combo game isn't exactly very easy for a beginner, especially one that isn't very used to short hopping and shffling in general. At the very most they can learn jab d-smash, use of projectiles, and other very basic stuff, but I'd recommend Marth and Sheik over Mario any day. Mario isn't a bad character per se, but they won't really improve much in terms of actually understanding the game aside from fireball camping.
