In reply to Esparta:
Lake District mountain trial. It's hard, it's tough, it takes you to obscure little bits of the lakes which other races don't. It's a classic. After that, stuff likes the Saunders MM is what I enjoy but none of them are big ridgeline/horseshoe style running.
Not all "running" is running sometimes it's just bashing through bogs and heather. If that's not what you're after then maybe mountain marathons aren't for you.
It depends what you want if you want a professional and slick event, I'd say Ourea events races, and Charlie Spronsons skyrunning events are probably where to look, if you are after a low key grassroots feel the lakes classic fellraces are where it's it.