6. Ferris (my reasons are well known at this stage) suits any gameplan
7. Warbuton - best openside in Europe
8. Denton/Heaslip - depends what type of game you want to play and what you want your eight to do. You simply want to steamroll the Aussies Denton if you want a smarter footballer and better linkman Heaslip. As a side note with the Lions scrum likely to do pretty well control at the base could be important. IMO Heaslip is the best at this as he has shown numerous times on rapidly retreating scrums.
20. O'Brien - not the best in any of the positions and not necessarily but the best option who can play all three backrow positions.
Right now I'm not sure Warburton is the best openside in Wales.