I'd recommend meeting with your sales team and discussing with them what leads they find the most valuable. Is it leads that have been exposed to a certain page or asset on the website? Or leads that have come to an event or watched a webinar? It always helps me to map it out (score-wise) before actually making campaigns in Marketo. This way you can see how scores would interact.
Usually I see higher scoring for events, webinars, and contact us forms. But this really does differ by company and your instance.