I think it's really impossible to even attempt to objectively say which is the "best" lounge in the world so I will list some of my favourites.

Favourite F:
1. HKG - The Pier (CX)
I thought I'd go out there and be different by saying I prefer the Pier over the Wing. For some reason I prefer the style of the Pier and the location in my "home turf" by the UA gates.
2. HKG - The Wing (CX)
No explanation really needed here.
3. LHR - Concorde Room (BA)
Simply for the exclusivity of being in the "real" F lounge rather than the BA Gold/OW Emerald lounge.

Favourite C:
1. NRT - Either of the NH lounges (International)
I've been to these two lounges many times on the way between North America and Asia. Love the noodle bar, attached smoking rooms and excellent staff.
2. HKG - United Club
Now I'll admit there's more food in the SKL and the furniture is very uninspiring, but the very open environment and the "This can't be a RCC" feeling are great. Sentimentally, this is my favourite lounge at HKG even above the CX lounges.
3. LHR - VS Clubhouse
It's just a cool place which epitomises the image of VS. It's not only practical, but fun to get stuff like a haircut done while you wait for your flight.

Honourable Mentions: HKG - CX Cabin (cosy, unique food/drink), SFO - UA IFL (need more like this in the US)