One that you'll do. The best exercise routine is the one you like to do. To find the best personal plan for you might require hiring a personal trainer. It depends on what you're used to doing and what your goals are. National guidelines for adults recommend a minimum of 150 min/wk of moderate-intensity or 75 min'/wk of high-intensity exercise and strength training twice a week.
A multifaceted one. Not only is some kind of aerobic exercise (like brisk walking or a sport you enjoy) important but some type of strength or resistance training will help ward off loss of muscle mass and probably bone density as you age. It's also very important to try to fit in some kind of flexibility and balance work (for example, weekly yoga or t'ai chi or simple balance and stretching exercises done at home.