Se we are all getting our free, random, level 80 legendaries out of cakes. The question is: whom to give them to? I went through the top three builds for each character on marvelheroes.info, and determined what legendaries were used. I then assembled a list of each legendary, and which heroes used them in the top three builds. Hope this helps someone.
"Other recommendations" is for legendaries that are not in the top three builds, but have been called out in this thread. The names that are bolded are those where the same legendary is used in all of the top three builds. Thanks, @doctorexcelsior
Casket of Ancient Winters
No one (Other recommendations: Storm, Iceman, Loki)
Cosmic Control Rod
Black Panther, Cable, Captain Marvel, Gambit, Human Torch, Iceman, Invisible Woman, Jean Grey, Magneto, Nova, Rocket, Raccoon, Silver Surfer, Star-Lord, Storm, Vision (Other recommendations: Green Goblin Energy build)
Excaliber
Black Panther, Blade (Other recommendations: Diamond Emma)
Golden Bow of Apollo
Cable, Cyclops, Dr. Doom, Ghost Rider , Magneto, Punisher, Rocket Raccoon, Storm, Taskmaster, Thor
Gungnir
Black Widow, Captain America, Captain Marvel, Cyclops, Gambit, Ghost Rider, Green Goblin, Luke Cage, Magneto, Rogue, Storm, Thor, X-23
Neptune's Trident
Dr. Doom, Iceman, Loki, Magik, Rocket Raccoon, Squirrel Girl (Other recommendations: Rogue Summoner)
M'Kraan Crystal
Deadpool, Emma Frost, Green Goblin, Iron Man, Juggernaut, Rogue, Star-Lord, Thor, Vision, War Machine, Wolverine
Norn Stones
No one
Savage Axe
Ant-Man, Black Cat, Black Panther, Black Widow, Blade, Captain America, Daredevil, Deadpool, Elektra, Hawkeye, Hulk, Iron Fist, Juggernaut, Kitty Pryde, Luke Cage, Moon Knight, Mr. Fantastic, Nightcrawler, Punisher, She-Hulk, Spider-Man, Taskmaster, Thing, Venom, Vision, Winter Soldier, Wolverine, X-23
Shield of Perseus
Colossus, Daredevil, Hulk, Nova, Rogue, Taskmaster, Venom
The Warlock's Eye
Cable, Dr. Strange, Emma Frost, Ghost Rider, Jean Grey, Psylocke, Scarlet Witch
Ultimate Nullifier
Hawkeye, Luke Cage, Mr. Fantastic (Other recommendations: Daredevil, Wolverine)