I sometimes come across creatures that I wish were Legendary so that I could build an EDH deck around them, either because I remember them fondly from a deck I used to play them in or because I think they'd make for a fun and interesting or powerful deck.
Some that come to mind for me are:
Ankle Shanker - Mardu aggro with this guy at the head sounds like a lot of fun to me.
Archfiend of Ifnir - One of my personal favourite cards, and an undeniably strong effect. Each creature. Each. Hot damn. I'd love to have constant access to him from the Command Zone.
Baleful Strix - no reason for this other than I just love the art and it would enable me to make the Dimir Nils Hamm deck I've wanted to make since getting into EDH.
I'd love to hear from you guys about any creatures that fit this bill for you for whatever reasons you might have; fun, power potential, sentimentality, whatever.