Space Marines: Oh thats a tough one to answer, I think the best I can do is break it down to three, which isn't bad considering there are thousands.
Space Wolves - big fan of Norse styled things in general, and the overall look they have is very pleasing. If they could get round to upgrading a few special characters or at least giving us a couple extra
HQ choices model wise I'd have an army. White Scars - Jaghatai is just that cool, and I'd love a biker army at some point, main reason I don't is I can't paint white well enough for me to attempt it. Scythes of the Emperor - always loved the backstory to these guys, ever since I saw the little bit about them on Advanced Space Crusade, my last go at fielding these guys as an actual army had a 'Captain' Mort at the helm.
Eldar: Iyanden, always loved the stories about them, Doom of the Eldar was a fave game for a long time, the fact they gained Yriel as a Special Character and he got a model, means I'll eventually return to Eldar, especially now my wife has Dark Eldar.
Orks: Death Skulls, Lyin' Cheatin' an Lootin' for as long as I can remember. It's getting to the point I am putting together/planning four to five 1500-2000pt forces that can combine up into a giant Apocalypse force if need be.
CSM: Thousand Sons, one of my first
40K armies was Thousand Sons coloured as they where on the side of the Space Marine (what would later be called Epic) box, which was red with a black M. I love the whole tradegy of the chapter and the fact if not for the arrogance/stupidity (too far?) of the Emperor then the whole Heresy outcome could have been very different with Magnus at his side. If they ever get around to doing a full Thousand sons plastic kit, I'll be making them again.
Imperial Guard: Part of me wants to name one of the female Guard companies from the previous book, but I have to go with the Necromundan 8th 'The Spiders' as I always name them in these kind of threads, and I'd love to give that army the time it deserves. To be honest as I am without my favoured Guard force
atm and will be for at least another year minimum, I may well start looking into a long term plan/goal of doing the regiment, even though it will require me converting pretty much every model in it.
Dark Eldar: Cult of Strife, just for Lileth.
Tau: Vior'ia I think it is, love the colour scheme.
Tyranids: Hive Fleet Kraken, just because of the connection it has to the Scythes of the Emperor marines.