Opinions are nice and all, but I'm compelled to post a definitive answer to your question. Compelled I tell ya, since posting this involves three of the greatest joys in life: using pivot tables, being a killjoy, and having the final word.
So first off is figuring what measure objectively compares league strength. There've been a few example of using game ratings here, and that'd be one way to go. Exceeeept for the practical matter that I have no way compile the rating information from the 9728 IV league games that have been played so far this season. I'm sure wozzvt has that info at his fingertips, but he's a fairly busy and crotchety man and likely can't be bothered with such mundane questions.
The only other objective measure to use is Country/World Rank. And when I say objective I actually mean "objective", with the sarcastic quotation marks intended. There's a bit of blind trust involved in relying on country rank as a measurement since no one knows exactly what it means. Wins, losses, months of supporter remaining on your account, number of bald guys on your roster...these are all things that may or may not affect your ranking. Personally, I'm think my early exit in the cup in season 11 is still holding me back. Not bitter, though...with a spot of therapy I've learned to accept and move on. As should you, since country rank is all we've got to go on here and I've got to go by something.
So, which league is the toughest? One way to figure that is to know which league has the highest ranked team. That would mean that the other 15 teams in that league have to beat out the toughest team in IV to advance to the next division. Certainly this would be the toughest task in IV? That team is "lakey"
(28954) in league IV.28, which is ranked #186 in the USA right now...higher than every other team in IV. To put that in perspective, the average rank of a III team is 249, and lakey is ranked higher than 56% of all III teams. The toughest league? IV.28. .
Fine. I can hear the chorus of "butbutbutbut" already. Screw the toughest team, you say...leagues have 16 teams and the toughest league is the one with the best assortment of teams 1-16. Every game is a dogfight, cream rises to the top, only the strong, blahblahblah. I'm on it. So lets take the rank of every team in IV and average them by league. Which has the best average rank? That would be IV.46.
(http://www.buzzerbeater.com/league/67/overview.aspx). The average rank for all 16 teams in IV.46 is 729. From #371 Vermont Catamounts
(22956) to the #1510 Wolf Wraiths
(28221) this league has the strongest teams top to bottom of any IV league
"You're a bloody grade A jagoff, fewmit", you might say. 1-16? Who gives a rat's ass that some crappy 8th place team is slightly better that another crappy 8th place team. Like the NBA, buzzerbeater is a tedious boring slog though the regular season until the real season - the playoffs. The best league is the one with the toughest
playoff teams. Okeydokey. Same deal...come up with the average for each league, but only for the playoff teams (1-4 in each conference). By this measure legue IV.28 is the toughest, with their playoff teams averaging a ranking of 382.
There you have it. 2 out 3 measurements agree: league IV.28 is the toughest IV league.