For Veterans: Kaisers.
For newcomers: You need to get your basics up and running. You should've rerolled into a 100% Eidolon, and then you probably want to get the opposing element MLB Eidolon with Orbs at the very least. So Fire-Thunder, Water-Wind and maybe Light/Dark (though Light vs. Light does just fine and so does Dark vs. Dark). After that, you need to decide whether you want a Kaiser for your 100% element or if you want to MLB other elements too. But that's a long, long, LONG road.