Here is a list of quality miniature ranges that suit fantasy and sci-fi wargaming in 10mm and 15mm scales. I only list figures that I have bought or seen and above all that I recommend and would buy myself.
Citadel Warmaster Range : These are by far the most detailed and exquisite models on the market. They are minute carbon copies of their 28mm counterparts, right down to detail and proportion. Unfortunately they are also the most expensive, but if you do choose to invest in them, then it will be worth it. Unfortunately they are no longer available from Games Workshop. You will have to buy them from Ebay.
Copplestone Castings : This range comes in a very close second to the Citadel Warmaster range. They may not be as detailed, but they are certainly perfectly proportioned, beautifully sculpted and well priced compared to Citadel. I fell in love with them the moment I saw them. The only downside is that the range is limited to Orcs and Humans with a Lord of the Rings look, but if more gamers show an interest in the range and scale; then Copplestone might just be persuaded to do more!
Eureka Miniatures : Another surprisingly good quality product for this scale, especially the newer ranges. It shows that they are constantly improving on quality as time goes by. Their older ranges are not bad, but as I said the newer stuff is of higher quality. I have just bought the High Elf Reavers with axes and they are excellent and perfect for White Lion warriors. Read the review on my blog.
Magister Militum : This company has a small range of Elves and Orcs, as well as cavemen and prehistoric creatures. The figures are presentable for their scale, but lack the high definition, excellent proportions and detail that come with the ranges mentioned above. Their range is rather squatish and comical, which is not at all bad if you are building an Orc army, but might not sit well if you have an Elf host. The up side is that they are relatively cheap. Nonetheless, I do recommend their dinosaurs.
Demonworld Miniatures : Ral Partha Europe has resurrected the beautiful but formerly defunct fantasy line of miniatures. Not only that, they are also producing and expanding on this line. I have heard of a new range of Lizardmen at 15mm scale. Personally, I think their dwarfs are the best they have to offer and I recommend anyone starting a 15mm dwarf army to first take a look at this dwarf range. Other races include orcs, elves, dark elves, barbarians and imperial humans, besides many more creatures.
Splintered Light Miniatures : Probably one of my favourite 15mm fantasy lines. My orc and goblin army is made up mostly from this range and they are sculpted by the brilliant Ben Siens. Ben Siens has also sculpted their dark elf and ratmen and I cannot recommend them more. This company also has some interesting wood elves and undead ranges worth considering. They are the only company I know of who may dwarf, orc and beatmen skeletons for undead armies!
Mirliton SG Miniatures : Mirliton have resurrected the old Grenadier 15mm fantasy range to our mutual benefit. These are the miniatures I had missed out on buying when I started to play fantasy at 15mm. Now I, and everyone else, have the opportunity to buy them again! Some of their ranges are a bit larger than others, for example their dwarfs are a bit bigger than the dwarfs of the Demonworld range, so I suggest you buy a sample and compare, or check out the review on my blog. Other ranges in their catalogue include undead, high and wood elves, and orcs and goblins. You can also download grenadier's old Fantasy Warriors battle game for free from the site. You can also buy this range from Vexillia Limited.
Khurasan Miniatures : Another great among 15mm miniature gaming companies. They are mostly concerned with 15mm sci-fi rather than fantasy, but their 15mm dinosaurs are excellent for lizardmen armies. I buy a lot of minis from khurasan for my 15mm sci-fi human army, and where sci-fi minis are concerned they are definitely one of the leaders in their category.
Rebel Minis : This company is one of my favourite when it comes to 15mm sci-fi. They have a variety of mecha, robots and alien races to choose from. On the fantasy side of things, they sport the Mighty Armies 15mm wargame with a whole range miniatures and races sold in box sets to choose from. My choice goes with the Wolfen sculpted by the legendary Julie Guthrie.
Critical Mass Games : Purely sci-fi, this company makes great miniatures at 15mm, including aliens, vehicles and infantry support. Highly recommend for any sci-gamer. Their minis can be used for their own game system or other independent game systems.
Magister Militum : This company also carry the Chariot and Blood Dawn fantasy ranges of miniatures. Both ranges are nice, but the Blood Dawn minis are more like 20mm than 15mm. They are way too big to fit in with the other rank & file ranges. Their orcs and lizardmen might work well for larger black orcs or kroxigors. I do not have any of the Chariot miniatures but they seem adequate.
Black Hat Miniatures : This company does not have any 15mm fantasy or sci-fi ranges, but they do have some nice looking 18mm lizardmen and other weird creatures that can be used in sci-fi and fantasy armies. definitely worth a look.
Blue Moon Manufacturing : This is a company I stumbled upon 3 years ago. They have some very presentable miniatures for 15mm sci-fi gaming. My personal favourite are the Orion Republic Heavy Infantry - of which I have an entire platoon.
15mm.co.uk : Their name says it all. One of the few sites I know that is wholly dedicated to 15mm wargaming. They sport fantasy and sci-fi ranges, but the quality varies considerably. I have some of their dwarf pikemen and axemen in my dwarf army.
Fighting 15s : This is where I buy the Oddzial Osmy minis for my human sci-fi army. Oddzial Osmy are probably the best looking 15mm sci-fi humans on the market. I cannot recommend them more. This company also imports the Eureka range of 18mm fantasy minis and the 15mm sci-fi Germans and Ventaurans. Worth checking out.
Hydra Miniatures : Relatively new to the 15mm scene, but they have started with a bang. Their line of Xenos 15 miniatures starts with the awesome Akarr. Superb little figures. I can't wait to see what their next race will be?
Plastic Soldier Company : Last year I was a convert to WW2 gaming at 15mm, and I was simply hooked. No turning back. If you need to start somewhere with building your Nazi or Allied army, then this company is definitely the place to start. They offer brilliant quality paired with excellent prices. Their platoon bundles offer even more value for money.
Clockwork Goblin : If you like weird world war 2 stuff, then Clockwork Goblin make excellent power armoured troops, Nazi zombies, walkers, mutants and mechas for the period. They are all compatible with the Plastic Soldier Company products.