All hormonal forms, whether the pill or Nuvaring (which I really liked for the first 2 years, so simple, no fuss, skin cleared up, no weight gain), eventually gave me such rampant and serious migraines (one every other day) that I have had to give up on the idea altogether.
So, I guess I'd have to say that the diaphragm was, for me, the easiest and most successful: barrier method, absolutely no side effects at all. And no, we didn't feel that It interrupted matters, so not inconvenient either.
However, if you have finished your family, I'd consider the non-hormonal IUD. I had one for a little while: insertion was not painful and was quite quick, once it is in, you can't feel a thing, no hormones to worry about.
It's a very personal decision and depends a lot on your age, health and personality.
Clare