He's hard to rate, really. He wasn't too difficult for me, but he seems to have been for a ton of others. All my pokemon were about 50-52 with only one being 60 since I was constantly rotating pokemon in and out of my team. My Ace, though, was Coalossal, who had a good Defense nature and it an absolute beast of a tank, so it took his entire team to try and take out my Coalossal since its speed was Maxed out after one Water hit. Without Coalossal though, I probably would have had a tougher time.
Against every other champion:
8. Lance (Strongest power team, but absolute easiest to beat since everything other than his Gyrados is super weak to ice and even that has a 4x weakness to Electric)
7. Kukui (Nothing terribly strong and the Sun/Moon AI was ultra dumb; all you had to do to avoid a potentially fatal Z move was throw out someone resistant and the trainer would never use it)
6. Steven (Another strong team like Lance's, but they were all garbage slow, so it was pretty easy to get the right counter in and take them all out. Only difficult one would have been Metagross, but even then it wasn't too rough)
5. Diantha (only good pokemon she had was Mega Gardevoir, but if you had a fast enough Poison type, you didn't have to worry about it. It was when you didn't that have one that she would become scary)
4. Leon (Decent strength if not carrying a highly resistant tank pokemon)
3. Blue (OG games were harder to level in and his team was pretty high leveled. Plus E4 was a tougher gauntlet if you didn't prepare right back then)
2. Cynthia (Garchomp was 4x weak to Ice but mega fast. If you didn't kill it on turn one and it Dragon Danced? Your team was wiped)
1. Adler (You could only fight him as the champion on the second run through the Elite 4 since team whatever interrupted, so his pokemon were all in the 70's and they were good pokemon. If you went in unprepared, you were gonna get steamrolled)