What is the most functional fitness exercises?
As Mark says... Rather depends what you want in terms of "fitness".
That means many things to many people. Want to run a marathon? That requires a different sort of fitness compared to someone who wants to do well with weekly pickup basketball games.
Want to prepare yourself for a trek in the Himalayas? You'd work differently than if you wanted to compete in a martial-arts tournament.
Fitness is multifaceted, and includes aerobic or cardiovascular efficiency, strength, power, agility, endurance, etc, etc.
For most, a simple, whole-body weight-training program, coupled with an aerobic ("cardio") component, will give a good baseline of fitness that can then be geared towards specific sports or activity-related goals.