What is the obesity surgery with fewest side effects?

Gastric banding. Gastric banding surgery for weight loss has the fewest side effects.
Depends. The band has the fewest short term complications but long term can develop problems down the road like slips or erosions. The sleeve has a slightly higher complication rate initially but few long term complications, most commonly gastric reflux.
Sleeve has fewest. The sleeve gastrectomy seems to have the lowest side effects. The only one seen on some of the 5year reports is acid reflux/heartburn (5-8%).