Depends on how you define best or worst, I guess.
Many guests will opt not to go to MK on a party day due to the 6pm closing time and lack of nighttime hours/entertainment. This generally manifests itself in “better” crowd conditions during the day, broadly speaking. It won’t be empty, but on the margins a touch better. I have found the rope drop advantage “lasts longer” on party days and we can cover quite a bit of ground.
Conversely, non-party days can be quite crowded, especially when you get into October where there are generally only 3 non-party days per week. That is not a lot of supply and a lot of demand in terms of days where normal nighttime entertainment is available.
This year, all party days 8/29+ will have 7am-8am EMHs and 8 am regular park openings. TBD if that added early EMH and early park opening change any of the party day “norms” (loose term) this year.