Smart Lipo was initially believed to be the least invasive form of liposuction surgery. With more recent developments in technology however, a more advanced form of liposuction was introduced called Tickle Lipo.
Unlike other forms of liposuction that produce heat and excessively damage the surrounding tissue, Tickle Lipo utilizes a safer form of technology known as infrasonic energy. Infrasonic energy breaks up the fatty tissue while providing maximum tissue respect. FDA approved Tickle Lipo is so gentle that it causes a "tickling" sensation and can be performed without anesthesia.
The answer depends on how you define "invasive." To me the least invasive is the technique that offers the least damage, swelling, bruising, and fastest recovery. Any such claims that one method is less invasive should be able to be documented with early after surgery pictures and movies showing the progression of tissues after surgery.
Check out such documentation here for my gynecomastia surgery which liposuction is always a component:
Beyond that invasive should also include how the tissues scar after surgery and move. Liposuction methods that involve exploding the fat cells such as laser or ultrasound seem to cause a great deal of internal scarring. Seeing how the liposuction sculpture moves after surgery is a testament to just how much damage has been caused. Here is one example of adhesion deformity after liposuction done elsewhere: