I'm curious - do you have a ballpark that you'd rate worse? The one other venue (aside from OCo) I've seen get absolutely trashed is Tropicana.
From purely a facilities standpoint, Oakland is the worst, although I've not been to the Trop yet (heading there in June though). But it's weirdly super fun to be there, even if it's a dump, so the experience of going to a game there is not bad at all.
From an overall gameday experience, I'd put Guaranteed Rate Field formerly known as US Cellular as the worst. The facilities are meh, very generic apart from the little pinwheels on the scoreboard, architecturally boring, and the fan base is dull. The best part of being there was the free booze we got because we sat in the "Scout Seats," their version of the Delta Club. The club itself was underwhelming in terms of food and atmosphere, but free booze helps cover up a lot of mistakes.