For Napoleonics,
Perry and
Victrix are probably your best bet as mentioned already.
Perry's range is more extensive, and has more factions such as the Confederation of the Rhine, however Victrix's plastics are cheaper, for example Perry's french infantry are £20 for 42 models, whereas Victrix are £22.95 for 60.
I'd also say Perry have the edge in quality but judge for yourself:
For the Civil War,
Perry have also got you covered, they've got a pretty solid range of plastics as well as a bunch of metals to flesh out the range.
For Dark Ages, there's
Gripping Beast's plastics which includes Vikings and Saxons as well as generic warriors, there's also
Conquest who make Normans. For other armies such as the Rus or Byzantines, Gripping Beast also have metal ranges for use with SAGA.
As for the 17th century, it rather depends on which wars you're interested in, the most popular are probably the English Civil War and the Thirty Year's War, both of which have plastics from
Warlord for their Pike and Shotte game system.