in terms of ease
bulk priming > carb drops/sugar scoop > syringe
the beer is only in the bulk priming vessel for well under an hour so it needs little more than a good rinse in hot water, very little additional cleaning in exchange for not stuffing about priming every bottle, nice even carbonation and not having to worry about disturbing the yeast cake while bottling. using an inline irrigation filter means you barely even have to worry about disturbing the cake while racking since clumps will be caught, and the filter can even catch any stray hops.
if all my fermenters were busy id probably use the sugar scoop, or maybe the carb drops (though i object to paying so much for clumps of sugar).
using a syringe just sounds painful, and i cant wrap my head around how you would make it accurate. you measure out some amount of water, then heat and disolve the sugar into it and now have an unknown quantity of water. how you'd split this evenly between a yet unknown number of bottles I cant imagine. I know I always clean more bottles than i think ill need, i'd doubt any two consecutive 23L batches ended up in the exact same number of bottles.