I think all the London lounges are excellent in their own way.
The T4 FIRST lounge is probably the worst out of the three. As an eczema sufferer with a phobia of water, the Moulton Brown Spa's are totally wasted on me!!!!

Consequently, I would say it was a draw between the LGW FIRST lounge for the view and T1 FIRST lounge for the Champagne bar and general spaciousnesss all round.
I clearly remember the BHX terraces being excellent and suprisingly large. However, I haven't been in a terraces lounge in the UK since February in London and May in Bristol....
I would agree with Fraser that the Berlin Terraces is excellent, however the toilets are alongside the Diners Club lounge.
I quite like the Admirals Club's in DFW with the LAX Admirals Club being the best of the AA ones I have visited so far.
In Hong Kong, I much preferred "The Pier" as opposed to "The Wing" - the latter felt too open and inpersonal, wherease the "The Pier" was darker with a more intimate feel.