Best for what purposes? I've been looking into this and it seems like there isn't a clear winner, especially if you include price points in your calculation. Different forms will give you different proportions of curcuminoid metabolites in your system, and I suspect some may accumulate in certain tissues more readily than others.
If you have a chronic problem, I'd say you should experiment with forms and doses until you find one that works for you. If it's for general health & longevity, I'd suggest going with a form which is not as potent and contains at least some full-spectrum turmeric.