From my experiences:
For * Alliance, The longhaul F on LH is very good and has good availability.
The F on SQ can be difficult for availability, as the new F suites are actually an R booking code and therefore not available on an F RTW ticket.
Thai is good, but TG often flies 2 class aircraft (C and Y) on alot of their routes.
It is almost impossible to get on SA for a RTW, even in business for an F RTW ticket.
ANA has an excellent F product on their 773s.