From a quality standpoint, I do think that the Japanese orgs have an argument. However, I will take issue with Strikeforce. While it is exciting, that is where it ends. It's an organization of UFC washouts. I may as well watch MFC if I want to see UFC flame outs (which I do)
Henderson v Babalu
Hendo went 3-2 in the UFC, then lost to Jake Shields
Babalu put together a nice post UFC run, but is 1-2 in important fights in Strikeforce
Lawler went 4-3 in the UFC and then got smothered by Shields
Lindland is 2-4 over last 6 and is more done then Liddell
Mike Kyle has losses to guys like Wayne Cole on his record (who? Oh, 16-17 record)
Antonio Silva is a promising HW
Look, I like Diaz, but he went 4-4 against the middle of the UFC WW division. His new punches in bunches style may give him a better chance, but really, does it matter?
I know you hate to recognize it, but the UFC is the main event and it's becoming a self fulfilling prophecy. Guys who make it to the big show probably end up fighting more conservatively because they want to stay there. They don't want to fight on a ShoXC event or in front 7146 fans (the hendo-Babalu crowd) on Showtime.
At least when I look at the Japanese cards I see a.) relevant fights in the lighter divisions and b.) true freak shows.