It's just a little something I was thinking about, but are there any 'must have' skins for the various races?
I think the various parts of Hangar-8's Golden Eagle Space Marines are probably a stylistic must, if not a fluffy must.
For the retro-fluff heads (the ones who remember 2nd Edition) then my Codex Apothecary and Librarian and Codex Chaplain are probably also a 'must have'.
For Chaos it'd probably be Hangar-8's Chaos Champion fix (unless 1.5 fixed the issue that the fix was for!), Hands of Fate's Chaos Sorcerer fix, Havoc's Colourable Sorcerer weapons, Hangar-8's Defiler (which, AFAIK, is still needed to fix the texture issue) and either Xen's or Mel_Danes' Chaos Lord for added spikeyness.
Eldar have the Aspect Warriors (or fixed colour Aspect Warriors for the complete fluff-nut) and possibly my Codex Craftworld Eldar. Some of the others look great, but probably aren't under the 'must have for fluffyness' category.
The Imperial Guard have several alternate camo's in the Imperial Guard Downloads section that could be considered 'must haves', but I can't choose between them. I did always quite like the vehicles in Soleil Rouge's WW2 inspired Camouflaged Tanks and Troops
Orks: As with the Guard, there's a few minor tweaks in there that myself and others have made (including my different shoulder pads and additions of extra bits to colour layers) but nothing 'must have'. Then again the Orks don't have a definitive look, so the definitive skin is always going to be difficult.
Necrons: The most obvious choice is Evil-Termite's Necrons with badges and banners. Quite why Relic left them out (or why GW asked for them to be left out) I'll never know.
Tau: As with the Necrons, one of the 'must haves' is the Tau with custom badges skin (this time by Adonis). Hangar-8's Non-teamcolourable Kroot are probably also a 'must have' for me. Although the Kroot can come in different shades, the shots I've seen of TC Kroot just go a little too far.
So, what skins do you think are indispensable either because they match the images in a Codex or because they're closer to what the fluff says they should be? Or even because of something they add in that Relic shouldn't have left out (like the Golden Eagles with additional detail and the 'with badge' packs for the Necrons and Tau)