I see what you're saying, but I disagree.
Whilst Kakashi may be in the top 20, Kamui anyone with speed feats or S/T jutsu can easily avoid Kamui.
It takes time for Kamui to warp objects as it's not instantaneous (see Minato vs. Obito, any time that Tobi teleports himself), and I doubt many will just stand there to let Kakashi activate his Mangekyo to get a shot at them.
Kakashi only has very few chances to successfully pull off Kamui. His stamina will be drained by using the Sharingan, allowing many characters to defeat him through sheer endurance if he misses frequently.
Not only that, but there are situations where Kamui cannot be used.
Shinra Tensei or Banshō Ten'in can affect Kakashi's stability, throwing his line-of-sight off. Susano'o, Sand manipulation, etc. might also be used to block the opponent from view, causing only the object infront of them to be warped away.
Large-scale jutsu, such as Wood Release Secret Technique: Nativity of a World of Trees, would force Kakashi to move and make it extremely difficult to aim.
Genjutsu, like Bringer-of-Darkness Technique that would effectively conceal the user, could also prevent Kakashi's success.
Long story short, Kakashi is far from #1. Kamui alone is not enough for him to be deemed powerful.