Whenever I play smash bros, the first thing I do is practice against AI for a couple hours to get a feel for the games physics, character move sets, range and timing, ect. Starting at smash bros melee I was rather easily able to 1vs3 the lv9 computers and win every time. I could also do it in brawl but starting at Smash bros 4, the AI finally got good enough that I couldn't 1vs3 them anymore. After playing them all, and even pitting them against each other just out of curiosity, I discovered that in smash bros 4, the AI handled playing Luigi the best, making him the hardest to beat, (all be it still easy 1vs1 at the time.)
Given that Smash Bros Ultimate just came out and has a large roster, it would take a substantial amount of time to figure out which character the AI does best with. I will say that, while I haven't lost 1vs1 to any of them yet, they are a significant improvement over the ones in smash bros 4 and can actually be challenging. All that said, I'm interested in some others opinions.