Fallen horde by Widowmaker
If any army gives scope for outlandish creativity then Chaos marines fit the bill. Fallen armies are often depicted as bridging the gap between what they were (Astartes) and what they are (Chaos) — Widowmaker's are firmly in the 'all-out-chaos' area.
Built from a mix of Imperial and Chaos kit sources, this is a nice collection of terminators, bikes, Daemon Princes and other characters. The assault troops using Sanguinary Guard models caught my eye — the addition of wings perfect to emphasize their fast-moving shock battlefield function.
They are of course in black (what else?) power armour — and have hand painted red DA Legion icons, as well as some having personal heraldry too. Colour palette is minimal, basing is dark so only the reds and metal details of the armour and weapons relieve the monotony.
The slightly blurry and over-saturated images adds to the miasmic chaos. All they need is Cypher lurking in their midst!