I have played Iceman a very long time with the following Legendaries maxed. Mikrann, Ultimate, Casket and Trident. I have found that the maxed casket is overall the best damage and quickest Legendary to dispose of Yellows and Bossed. But for some reason it says the Mikrann is the best if you go by Green and Red Triangles. Going to max out the Bow next. Also, Instead of throwing points into Iceman's Energy, Put it into Mind= Crit (First Option at Top)
If you do decide to go with the more raw damage with the Casket, Make sure you grab gear with all the Crit Hit, Crit Damage, Brut Damage and Brut Strike. It will take a little longer because this casket is missing any crits or bruts.
Filipino Good Girl!