1. Daemon Prince by Juan Diaz
By a landslide.
2. Gal Vorbak
Love 'em. Gotta buy some more.
3. Terminators/Terminator Lord
In a bit of a cop out I'm gonna lump those two together. Even if they're beginning to show their age, these terminators provide the basis for most of my conversions for my Night Lords and I usually try to preserve as much of the original as possible; all for a reason. The trims are not excessive, the heads and weapons look great and the minis as a whole just convey this archaic, brutish vibe that I want in chaos marines. Also, their trophy racks come in handy all the time. They get bonus points for easy customizability, which newer units often lack.
4. Raptors from 2002
While their newer incarnation has a few good bits (the chainswords alone justify the buy), I like the older ones much more. They weren't designed just as marines with jump packs and instead look like avian predators hunting for prey. Their helmets, clawed feet, shoulder pads...it all fits so well together. Oh and the jump packs! Still the best.
Maybe they could stand to look more vicious and intimidating, but after finishing two of those good doggies I came to appreciate them a lot. I left off the lasher tendrils and magma cutters and switched its head for the forgefiend one, because I thought they were either overly burdening its frame or because I didn't care for their design, but other than that the mini is snappy as hell. Daemon engines were a great addition to the chaos roster and I hope GW releases a couple more.
Special shout-out to the aspiring champion from the box of chaos marines back in 3rd/4th edition with his awesome power fist, plasma pistol, and horned helmet; and to obliterators. I know they look pretty terrible, but I was quite excited when I first saw the teaser for them in White Dwarf. I love converting them and getting the most out of those hideous bastards.