My 15mm WWII collection consists of the following ranges, Battlefront, Command Decision, Essex, Forged in Battle, Peter Pig, Plastic Soldier Company & QRF (don't think I've forgotten any) I don't feel any one manufacturer could provide all I need, either the range is too small or quality is not consistent. There are pros and cons to most these ranges which I will give a brief summary of;
Battlefront; huge range much of which is excellent but can be pricey (look for offers discounts) and some sculpts are particularly poor e.g. stranglely proportioned infantry.
Command Decision; another huge range cheap and generally good. I get mine in the UK from Skytrex and have not experienced issues with miscasts, seems this is not so much the case with Old Glory though who also cast and sell the range in the US. I do find some of the vehicles need a bit of work to fit together though.
Essex; only a few infantry in the range but they are all clean well proportioned castings. Not the cheapest but reasonably priced.
Forged in Battle; rapidly expanding value for money range. Vehicles go together quickly and easily, some people don't like the cast on bases but I do. Some silly errors such as MG loaders on the wrong side and lack of Bazookas for US.
Peter Pig; large range of infantry and quite a few vehicles. There is something about the sculpting style of the infantry which I'm not too keen though.
Plastic Soldier Company; another expanding range providing excellent value for money. I was put of at first about building kits but now I've tried I find they them quicker and easier than metal/resin.
QRF; another large range, some of their stuff is a bit rough and ready, but newer sculps are excellent. do a lot of stuff others don't.
Going forward I will be using PSC as first choice for vehicles and FiB for infantry as a general rule, but I will consider all the available ranges when looking to create a particular unit.