Depending on what the person you are teaching wants to do, this question can be answered with any class in the game. That's problematic for choosing, but here are a few suggestions:
Fighter. This is the boring option, but it actually comes with a ton of chances for improvisation. With simple "I hit things with my sword and I'm an awesome athlete" class features, you are combat effectively right away. One of the benefits (and potentially detriments), is that you don't have much cool stuff to do that is immediately codified in abilities or spells, a lot of your utility is up to you. That can be good or bad. Pros: easy to learn, easy to play. Cons: making it fun may require too much improv for a new RPer.
Rogue. Marginally more fun. Much more codified out of combat utility, and you get to be sneaky which can be way fun. Two equally viable alternatives in ranged (provided you play a halfing or have good access to terrain you can hide in regularly) or melee (go two weapon fighting with another melee class and you've got easy SA dealing). Pros: More to do, tons of codified out of combat utlity, considered a fun class. Cons: more work to play it well (you have to think about positioning to get your SA off).
In D&D 4th edition we talked a lot about funneling new players into the game with striker classes. Things like the sorcerer, blaster wizard, rogue, ranger, were classes where you could do a lot of damage (fun!) and do other cool stuff (usually skills etc). The equivalent classes in 4e come in a far greater variety of complexity (Ranger and Monk are both casters Rogues and Fighters have that option. Casters are much more complicated in 5e than they were in 4th). So I would definitely recommend taking a martial character as your first one if you want something low maintenance.
However, nothing is going to be a substitute for sitting down and coming up with a character concept together. Have the person you are working with describe the character they want to play. If you can, match it up to an existing class, if you can't, we might be able to help with that specifically.
Coming up with a character someone wants to play isn't necessarily about identifying an easy to play character (although that can be helpful), it's mostly about identifying what they want their character to be able to do and then designing a character (together), that fulfills that criteria.