I guess the hard part would be to define 'regular weightlifting' but i guess it should include anything not related to the other styles mentioned.
Define what lifts constitute doing "powerlifting" vs. which ones you consider "olympic lifting"....and what do you mean by circuit?
A combination of styles would be best...it would give u more choice and help avoid plateauing. At least that what I find.
typical powerlifting would consist of bench press, squat and the deadlift as the mainstays.
with olympic lifting, the mainstay exercises would be the clean and jerk and any of the variations of that exercise.
circuit--high rep, low weight, low rest periods.
Hmmm I'm stuck on Olympic and circuit....I guess it depends on how much cardio you do. If you do sufficient cardio then I would say Olympic is the best, but if you don't do that much cardio I would say circuit is better. All depends on who you ask, but IMO you want to achieve the perfect combination of speed, power, and conditioning.
Personally I don't think you can generalize. There are so many different factors.
If you had said "You need to gain about 15 lbs for moving up in weight class, and in the process you wish to build strength without losing flexibility... which would you choose?" Then I would have a better answer for you. Unfortunately with so many styles and so many weights and so many other factors to narrow it down into one "best" lifting technique is cutting yourself short IMO.
There is also time restraints, and any other number of factors.
I'd say power lifting with a hint of isolation training. Power lifting is good for your large muscle groups, as well as their stabilizers. If you if you're looking for specialty training, do some isolation exercises as well.
I went with "combination" but the emphasis IMO should be on powerlifting type exercises. But exercises like pull-ups, shoulder presses and heavy dumbell rows are also useful for building overall strength and working motions that are useful for fighting.