Not sure of your price range but I'll give u a few sugestions -
Ron Zacapa Solera 23 (Fantastic Solera style Rum from Guatemala)
Diplomatico Reserva Exclusiva (Great Venezualan Rum)
Ron Atlantico Private cask (Another award winning Solera Rum from The Dominican Republic)
Dictador 12y/o or 20y/o (Really sweet Caramel heavy Solera Rum From Colombia)
Flor De Cana 18y/o (Not so Sweet but brilliant all the same From Nicaragua)
If Your friends havent tried these then I'm sure they would get them talking
Not sure where you are based but if you are in London you could try visiting Gerrys, Amathus and The Vintage House they have a great range of rums. Also check out thefloatingrumshack.com and their friends link for other rum ideas