Revenant - the spooky Eidolon frame. His quest is fun and gives some good insight into the background of the Plains of Eidolon, but more importantly, he's a fun frame that is absolutely ideal for both solo and team play. Seriously, you can't go wrong with Revenant - unless you're trying to do the Tridolon...

Have a listen to why you need to have a Revenant in your stable.