It depends on the definition of "best" - i.e. best for what.....??
I've owned my F6 for about 4 years. I have the battery grip and the camera is absolutely brilliant at everything it does. However, whilst it is very solidly built, it isn't as solid as my F5 - but then, the F5 doesn't allow the battery compartment to be removed as a "lightweight" option. The F5 does, however, allow different "heads" to be used - whereas the F6 doesn't.
In terms of the shutter, I'd call it a dead-heat. Metering is also too close to call, for me, as they are both incredibly accurate. Mind you, I tend to shoot B&W film at half box speed (+1 stop) all the time and it's difficult to get it wrong enough to have that be a problem. I tend not to use flash, so I can't comment on that sort of whizzy stuff.
Both F5 and F6 feel extremely well-balanced - especially when using large, heavy or long lenses (I use the 14-24mm/f2.8, 28-70mm/f2.8 and 70-200mm/f2.8 Nikkors and they all feel very comfortable). A good example of an F5 (now out of production) can be had in the UK for £200-250. A nice s/hand F6 will cost up to three times that amount (retail prices) but the cameras will last and last...
These cameras aren't for everyone. The F5 was definitely aimed at professionals. The F6 was an exercise in making the best possible 35mm film SLR but I've seen quite a few pros use them.
These are not cameras for
stealthy street shooting - though I do shoot street stuff with mine, as well as travel, landscape, etc. Neither are they for anyone with a bad back or weak wrists. It's results that count, however, and when I want something to be exposed accurately and as sharp as it possibly can be, these are the tools to use. Then again, I am a bit of a control freak.....