This app was mentioned in 2 comments, with an average of 1.00 upvote
For Gameboy, I use John GBC. The free version has some ads, but it's pretty cheap to buy, and I thought it worthwhile. The button positioning is good, they respond visually so you know they're pressed, and the games run smoothly without sapping too much battery (On my Samsung Note 5, I feel safe playing at 17% battery for the next hour or so).
I can't imagine playing N64 or playstation games on a smart phone. Just trying to play Link to the Past drove me nuts!
Hmmm weird. I know it has IAPs, but I think it's free to download. At least it was for me.
Ninja edit: This is the free link, OP: https://play.google.com/store/apps/details?id=com.johnemulators.johngbclite