Which are the best denture types to fit where a couple of missing upper molars are now removed under a bridge due to tooth infection,chrome cobalt or semi flexible?and also best for quality comfort and durability? and is there other newer types?

A chromium cobalt frame will provide rigidity and if properly designed will not cause aberrant forces of your abutment teeth. A flexite denture distorts when you bite and the forces on your remaining teeth are unpredictable, often causing premature loss of those teeth. If your budget allows, consider root replacements (implants) to restore your mouth to proper form and function.

The best person to answer your questions is you General Dentist, the practitioner that best knows your mouth. The condition of your bone, gum tissue, and adjacent and opposing teeth will determine the restorations of choice for you, and can only be determined after examination, x-rays, etc. For most, restoration of choice is implant/crown. See your Dentist.