The designs were a bit overdone, could of been better honestly but I like most of them. I think I hated Heatran's design though XD;, it looked rather ugly and had a squashed sort of face. Also it sort of had this scary look in its eyes, I never even really liked it as a Pokemon honestly. :P
It has a interesting type combination but its speed and 4x weaknesses are really frustrating and annoying when you're trying to work with it. Honestly I think Heatran is my least favourite legendary. :B. I loved the Shaymin forms but I suppose they could of scraped Shaymin and kept its sky forme.
I don't honestly like creepy looking Pokemon and the plot behind Darkrai was rather scary but it's a badass looking Pokemon and the events that happen before you can obtain it are quite interesting :x. Also I think the best legendary design-wise in Gen 4 was honestly Giratina. It looked ghastly and mysterious, it just looked amazingly sleek and badass. I loved the thing. <3