Well this could be a long post as i have 6 diffrent armys.
Eldar
Spesifically Iyanden force. I chose this as my ver first army as i have always love the Wraithguard modals as well as the Wraithlords. And if i was gogin to spend £77 on a unti of 10 i'm goging to bloody well use them! I had always loved eldar and i have to admit they are what got me back inot 40k and learning to play it after i got a box of Fire Dragons just to paint for fun. It soon expanded from there and is currently my largest army. Becuase of this there is very little i don't have for my army with Harlquins, Dark Reapers and Shinning spears being the only ones i don't have any of.
Space Marines
Originaly started out as my own chapture but after a while i got board with the franky bad colour scheme so i had changed to Imperial Fists. This army was actually got becuas eof it's versitilaty which Eldar didn't have. It is ofcouse the "most popular" army and thus being able to use and know them proved to be a big advantage.
Tyranids
This army is very much small that the moment and i got it becuase i love alot of the modals and it ment i could run a horde army which Marines and Eldar can't do. I run them as Hive Fleet Medusa as Tyranids adapt there colour to there enverioment and they fought on a snow world agianst and won agianst the Imperial and Space Puppies (I dislike the puppies). This army needs alot of work at the moment and it's swam ability are only jsut startign the serface.
Blood Angels
They originaly started as me being lasy and the orignal colour scheme of my normal marines ment people oftne mistook them for Blood Angels. After a while i desided to take some of my "combat happy" aspects form my Marien army and turn them into the Angels Encarine. This army is VERY small as the only modals i have to for are 5 Assault marines, 8 Death Company (Old metals one still needing conversion), 2 Landraiders (redeemer and normal), 6 Bikes and Seth. so it's bearly legal. This army is due for an overhall soon (in the new year about feb or march) but until them i just proxy my Imperial fists as assault mairne sif i need to.
Chaos Space Marines
Mainyl got them as i love the Sorcerer and felt i needed a shooty army as every other army was very much clsoe range or CC. It turned out much mor eof a magic army with alot of tzeetch about and i have to admit that i possibly have the most fun painting thsi army over all the others. I have only just got it to a good size with minor works needed here and there but i really enjoy them (Just look at my "kill count" for Gift in my Sig). my army is actual the Bleak Brotherhood.
Imperial Guard
This iof my true Shooty army but also combines horde as well (I don't use many tank in any of my armys). It is VERY small being at 630pts but as one person at my local once said "Guard are great but you need to take out a loan to get a good army". He is compeltly right

I'm waiting to see what i have after chriatmas but i will try and get them to 1000 by Feb. My free hand contiunes to screw me ovewr with them as i wanted to paint red Cobras on them as they are the 146th Catachan devision "the Red Cobras".