What you need is a synthetic-fill parka that's got as much loft as possible. For really, really cold weather, that will need to be supplemented by considerable layeringexpedition-weight long underwear, maybe a mid-weight fleece layerbut a good top layer is a start to keeping warm.
Several pretty good choices. One is Patagonia's DAS Parka ($265; www.patagonia.com), a full-cut (room for layering), hooded parka that's stuffed with Polarguard 3D for reliable, durable insulation. It's not long on fashion, and comes pretty close to down-parka warmth. On the expensive side, but worth a look, is Arc'Teryx's Fission Belay Parka. It uses PrimaLoft for insulation, which is a little softer than Polarguard so feels more natural when worn in a jacket. And it has a Gore-Tex shell so it's both warm and waterproof. On the downside, it's expensive at $500 (www.arcteryx.com). Lastly, The North Face's Morpheus Belay Jacket ($349; www.thenorthface.com) offers similar performance with a proprietary waterproof-breathable shell and Polarguard Delta insulation, so it comes in at a lower price.
Any one of these is about as warm as a non-down jacket can be. My first choice probably would be price-based, so I'd look first at Patagonia's DAS Parka.
No idea what you're up to in planning this theoretical Arctic standout, but whatever it is, keep warm!
For more winter-worthy top layers, check out Outside Online's Jackets Buying Guide.