|Edited: 03/02/05 21:28
This depends on what you would like to do with it. Do you want to fly it yourself and never use the ship for anything else? If so, ignore the OOS section, if not, try and balance both uses with your selection.
For Personal Flight the popularity is roughly:
The Osprey has an impressive weapons loadout, which includes the option for 4 BPSG and 2 GHEPT, the highest shielding, the second best effective cargo hold and the best steering for any M6. It is the slowest M6 though, and therefore isn't the best for impatient pilots.
The Dragon comes second, but it is the opposite type of M6, fast, manoeuvrable, obviously intended for hit and run tactics against ships, but can be used to devastating effect by an excellent pilot. Its drawbacks compared to the Osprey are a third of the shielding, no PSG capability, and a smaller cargo hold.
The Centaur and Nemesis are third, but this is due to the fact that they are both middle of the road M6's. The Nemesis and Centaur are quite similar. The Nemesis has PSGs. The Centaur has more guns. The Centaur has a decent cargo hold. The Nemesis has a bit more speed. They both have average M6 shielding.
The Hydra is last, mainly because it is a jousting ship. It is too fast and nowhere near agile enough. It has the biggest effective cargo hold (because there's fewer guns and shields in it than the Osprey), but the least effective guns of all the M6's. It doesn't fit at all with the other M6's. Think of the Hydra being more of a, well, secure trade ship. Few things could kill you, but you'll have problems killing anything.
For out of sector (OOS) the popularity is roughly:
The Centaur is cheap, it has the minimum required shielding of 250Mw and it has BHEPT (or greater) capability. This is all that's truly required.
The Nemesis is also fairly cheap, but due to the small cargo hold, will take longer getting to its sector. It comes a close second, being held back by the absence of one gun mount.
The Osprey is expensive to fully outfit in terms of both monetary cost and in the set-up time. It would have ranked first if not for those drawbacks.
The Hydra is very expensive to buy, but had it been cheaper it would have taken third place due to it being adequate for the job and not taking anywhere near as long to set-up as the Osprey. Don't be afraid to use captured Hydra's for OOS defence.
The Dragon, well, it has got guns, but it doesn't have more than 125Mw shields. This, in OOS, is unforgivable. The Dragon will be destroyed long before any of the other ships would be.