Mud. I'd rather not. . . but I always feel great at the end of a ride all lathered up in sweat covered in yuk head to toe. Makes me feel like a mountain biker. . . a real one.
Riding in mud will introduce you to chain suck. It will help you understand the limitations of your tires and brakes. It will improve your bike handling skills. . . descending on a slippery trail. FORCE you to clean your bike, especially your drivetrain. Get you out of the house when you'd rather watch another episode of Rosie. If you race, or want to race, it introduces you to conditions you will see in a race.
You will ride slower in mud. You are more likely to crash. You may have to hike-a-bike through the worst of it. But, it's part of mountain biking.