Adler have excellent infantry – on the large size (more like 7-8mm) but excellent range and poses
GHQ vehicles are the best there is, but expensive (can be ordered from Magister Militum in the UK)
CinC are similar to GHQ but only US supplier I think
Heroics and Ros – large range but small with basic detail. Cheap though!
Irregular – quality varies but some items are good, very good for terrain stuff.
Scotia – limited range, some useful items.
Baccus – new ranges, infantry only at present but good poses.
Really depends how you want to balance cost v detail.
I have mostly GHQ vehicles and Adler infantry with a few odds and ends from the others , and have also bought some of the new baccus figures to try out.
Interest include 6mm WW2, 6mm SciFi, 30mm Old West, DropFleet, Warlords Exterminate and others!