Jiujitsu is a highly competitive grappling sport and a staple for MMA. It's rare you'll find an MMA fighter who doesn't study Jiujitsu. Usually when you do it's somebody who has mastered wrestling or Sambo or some other type of grappling art that gives you a significant amount of grappling experience already.
Many people have different opinions on what's the worst and best positions in Jiujitsu. Some people prefer guard while others prefer North-South, which both are very popular as is. I'm no expert and keep in mind much of this is just personal preference, but in my experience the best position is Side Control and the worst is Full Mount.
The reason I say full mount is the worst is because all of your weight is centered in one area. Any good Jiujitsu practitioner knows that it's easy to reverse a full mount. Trap an arm, trap an ankle, extend your back and roll over to be on top of their guard. People like to go for the full mount because of it's ground-n-pound capabilities. I avoid it in Jiujitsu because of how easily you can be toppled over and reversed into a less dominant positon.
Side control is mine and many others favorite positions to be in per grappling. Side control allows you to displace your weight across the ground while half your body is on top of your opponent. Successfully pinning them to the ground. It opens you up to a broad array of submissions. You can go for Americana, Key-locks, arm triangles, neck cranks, and so much more.
Be sure to find which style of grappling suits you and what positions your most comfortable in. Let us know what you think. What is your most and least favorite positions in Jiujitsu or grappling?