for best isolation I would say that custom (well done) and with full shell(empty shell is not even as good as foam) would give the best isolation and comfort. what made me go all foamy again is that with customs my own body noises are transmitted much louder than with foam (breathing, moving, heart beats,stomach fun) and that was very distracting for me. the better the isolation, the more you get annoyed by your own body.
whatever I have tried, the less distracting for me have always been with music. white noise has too much regularity so I get used to it and it doesn't cover sound that much.
just passive isolation even if it goes to -35db won't ever come close to -20db+40db of music(super quiet music).
I use some classical piece of music played in repeat1 and found that it was my best way to concentrate (I strangely think better with classical???? dunno why but that's a fact for me).
about active noise canceling, I've never tried one that came remotely close to some etymotics. it's brilliant for low frequency noises, but against people talking active noise canceling is crappy. there was one that impressed me, it was a fullsize headphone for helicopter pilots. just the passive isolation was impressive, and then the active noise canceling removed remaining low freqs. I though it could be great as it's easy to remove when someone talks to you, but the clamping killed me after 1mn :( . it was even clamping harder than a hd25.
in the end I use hf5+foam+classical. cheap but it can be itchy for long use everyday, so I'm still open to suggestions ^_^.