Scale, weigh daily, average weight loss for week.
Daily weighing can be a great motivator but it can also be a disappointment to see the scale bobble up and down. Keeping a record of daily weight and then averaging can give a truer picture of weight loss and keep negative feelings at bay.
Scale weigh weekly or monthly.
Many folks find that daily weighing, even with averaging, is just too tough. For these types of folks weighing at the same time, each week or month, under the same circumstances helps them stay on track and not obsess about the numbers.
Tape Measure, chest, waist, hip, upper thigh, upper arm.
This is recommended even if you choose to weigh. Often plateaus in weight can occur but inches are still lost. Keeping track of lost inches is just as important as keeping track of lost pounds.
Reference clothes.The use of clothes a size or two smaller than ones current size can be "the best way" to keep track of weight loss if you are the type to obsess about any numbers, even those on a tape measure. Reference clothes are also nice because inches can be lost in places that are not measured, whereas reference clothes remain constant. There is a special kind of feeling of accomplishment zipping up a pair of jeans that could only zip up half way just the week before.
Choose the method of measuring success that works best for YOU. The one that keeps you on track and not eating off plan is the best one for YOU.