I would start one of the TOS films--probably TWOK or TUC. Why these? They both feature cast members in their element--aboard the Enterprise
and dealing with an impossible situation--and contain a nice mixture of drama, action, intrigue and characterization.
TVH would also be a reasonable choice, but it's a lighter film than the others and is hard to take seriously in parts. If you decide to start with the TNG films, I'd suggest First Contact