I feel that the Omni that surprises me the most is the Marvel Horror one. That is a good showing at 900+ units
The Timely Sub-Mariner one also feels pretty strong being a Golden Age collection and about in line with some of the Bats/Supes GA volumes.
The one I'm not surprised by is the Spider-man by Byrne Omni. That run of books is not viewed with any fond nostalgia and seemed an even odder choice as a collection despite being Spider-man than some others they'd surprised us with(Hellstrom for example).