This is such a sticky debate, as, in the end, everyone always thinks that their regional is the toughest, because everyone else doesn't understand the tough rivalries attendant at their regionals, and how hard it is to get out.
Technically speaking, the toughest regionals would be those with the most bids, because, bid level is equated to field strength, so, the tougher the field, the more bids.
However, a bid poor regional would argue that their regional is, in fact, harder, because it has only 4 or 5 total bids, compared to a regional with 7 or 8.
I don't know what the best measure is, other than to say that all of AMTA keeps getting better, stronger, faster, it is pretty tough to get out of anywhere anymore, and, with the advent of super regionals next year, the larger question will be "who has the toughest super regional" rather than who has the toughest prelim.