While I haven’t ridden Serre Che on a bike, I ski there and it’s a lovely place. From what I’ve seen, there’s some awesome trails to be had! http://www.serre-chevalier.com/en/summer/activities/cycling/bike-park/ You can lift out of Briançon or any of the other base stations by the looks of it. Chantemerle would be my pick. Some great food on the mountain too, especially at Le Bercail at Aaravet.
As above, Sauze has a good bike park, and some of the trail features visible through the snow looked very good. Also, at Orso Bianco at Sportinia (mid-station) they serve the best pizza on the planet.
Monty? Well, not skied or ridden there, but drive through often enough. Looks OK.
La Grave? Great for shredding da phatt pow, but not so sure it’s set up for bikes in the Summer.