After having done around the world trips on all three major alliances in the last year (for business), I thought I would briefly highlight the plusses and minuses for each alliance from my experiences.
I am currently top tier is Sky and Star, and should be second to highest tier in ONE after the miles from this last trip register. I have no dog in this fight, this is just my experiences.
SKYTEAM: (ticketed through DL)
Best computer integration between the member airlines.
Good crediting of miles flown on member airlines without asking.
E-ticketing even on trips more than 16 legs (although re-issue is required part way through trip).
Poor coverage in SE Asia.
Some difficulties gettiing seat availability Transpac in Biz.
No international F on most ST carriers, so a RTW F class ticket is almost all Biz class.
STAR: (Booked through LH)
Good access to LH RTW desk with very helpful (Canadian) staff.
Good global destination coverage.
Reasonable computer integration between member airlines.
Printed paper tickets by mail for trips more than 16 legs.
Good RTW trip planner online.
Tried to book through UA first, but could not even figure out how to talk to an agent, so I gave up and called LH.
Some member airlines poor at providing inventory (SAA).
SQ does not provide F space on some flights (A-380...).
No e-ticketing of 16+ leg trips.
ONEWORLD: (booked through AA)
International F available on most longhaul flights airlines in the Oneworld alliance.
Good global coverage.
Easiest to book reservations, no problems with seat availability.
Poorest computer integration between member airlines. Miles don't automatically post and computer reservations are lost.
More than 16 leg tickets must be HANDWRITTEN at the airport.
Having to connect through LHR/LGW stinks.
Poor AA domestic network connections from my home base (not important for others).
From the above, you can see that each has plusses and minuses. I don't know which I will use this fall for my next RTW, it will depend on the destinations.