I think the thread would be more interesting if we also said *why* we think our dark horse is a great cartridge.
My list goes as follows (and not necessarily in order of my preferences):
6.5x55 - When I managed a rifle brand, I kept this cartridge in the lineup, in spite of the fact that it never took off (and I'd hear it from my boss at every year-end report meeting). It just felt right, somehow. I now see that the new PM has drastically curtailed the 6.5 from the offerings--which makes sense from a marketing standpoint but still breaks my heart. I must confess that I never understood why Americans don't buy this cartridge. There are few cartridges between .25 and .270/.280 that are as mild, accurate, powerful and versatile as the 6.5x55 which, with the right bullet, can span the gamut between varmint and moose (ask the Finns). Why it is not more popular, especially in light of America's love affair with surplus Mauser 96s, I'll never understand.
.280 Remington / 7x64 Brenneke - Bridges the gap wonderfully between the 7x57 and the 7mm Rem Mag, giving you more punch than the former without the pyrotechnics of the latter. Especially with heavier bullets--like the wonderful 170gr soft at 2700fps--it could be a perfect, mild-recoiling one-rifle battery for America's big game (and much of Africa's too). I'd go as far as to say that the .280 Rem is a better cartridge than the .270 Win--all else being equal, more bullet-weight selection, and much better sectional density (therefore penetration) with the heavier bullets. I've never understood while this most excellent of .30/06-based cartridges has never taken off, while most others have. Perhaps no Jack O' Connor to engrave it in America's heart...
.300 H&H - With its lovely sloping shoulders, this most ancient of .300 magnums feeds like a dream, is a joy to handload and has plenty of history to boot. It is inherently accurate, can be loaded down to .30/06 levels or up to match its more powerful cousin the .300 Win Mag. But most of all, its long neck just begs for it to be loaded with 200+ grainers, making it a surefire killer for bigger game. As soon as I get a rifle in this chambering, I will load it up with a 250gr Hawk round nose to duplicate the .318 Wesley Richards (another awesome cartridge).
.303 British - While sales of .303 remain brisk for the zillions of Enfields out there, I am baffled as to why this cartridge is not loaded in more commercial rifles. I was happy to see Ruger offer it in the No. 1 (matter of time until I get me one), but I also think it would make a great chambering for a magazine-fed lever action like the Winchester 1895--which has been an on-and-off reality without ever becoming even a mild favorite. The .303 offers plenty of power and trajectory for any hunting out to 300 yards, and with its original 215gr bullet it makes an interesting cartridge for anything that walks in North America.
.35 Whelen - What I said about the .280 with regard to the 7mm class applies to the .35 Whelen for the medium bore class. What an awesome cartridge this is--powerful, mild-recoiling, flat-shooting, .30/06 based and therefore easy to chamber in an existing rifle/action, and yet it still is regarded pretty much as a wildcat. Come think of it, the whole non-magnum category between .300 and .375 is fairly underrepresented in popular bolt-action cartridges. I couldn't think of a better one-rifle battery for any nondangerous big game than a .35 Whelen and a few boxes of 225gr spitzers and 300gr round noses.
Besides my personal pets (obsolete British African cartridges, some European ones), I really do think the above to be truly great chamberings that have suffered from the magic of a popular "champion," good marketing and/or a smashing new rifle chambered for any of them.