Bit of a tie between IV and V for me. I dun really like any legendary in IV except Giratina and Darkrai (pretty much one of my fav overall) and while V has much more likeable ones, the kami trio is just super lazy. Sure they got new formes but they should've been that originally and not too forget being available without having to have a 3DS and buy another game.
So yeah, V is butthurting and IV is meh. (and III is forgetable)