Naturally, people will respond that some of these cars are not sleepers, not mundane lookers, not wolves in sheep's clothing, but they are. It is not a slight - I love them more for their benign looks. Admittedly, the list is slanted a bit towards Mercedes but there is no denying they were masters of building viscous muscle cars in sedate packages.
Mercedes 450 SEL 6.9
6.9 LITERS. That is a mighty V-8. Absolutely bristling performance with gobs and gobs of torque. Not convinced that a car of this size could be a thrilling drive? Check out Ronin.
Take the dowdy W124 Mercedes, build half in the Mercedes plant then shuttle it back and forth a half dozen times to Porsche. Only to end up with a 150 mph Mercedes that looked nearly identical to your neighbor's more pedestrian 300e. Makes perfect sense to me, but then I didn't shell out the extra $30k or whatever it cost over the more sober 300 series. Though Mercedes still builds blindingly quick sedans, the days are over when they didn't advertise it. For what it costs to maintain a vanilla Mercedes, I suspect total cost of ownership would be about equal between a 500e and a brand new Porsche Carrera.
GMC Syclone Pickup Truck
Ah yes. The Euro look of automobiles circa early 90s. Body color plastic skirts and airdams were added to every grocery getter in Detroit but the Syclone actually had the performance to back it up. A turbo charged Vortec V-6 with AWD made for some very impressive quarter mile times in the mid-13's. I dream of finding a totaled, insurance auction Syclone that was hit from behind. Chisel off the go-fast body molding, replace the wrecked bed with a stakebed, dust the whole thing with a can of Krylon primer, fill the back with lawnmowers and weedeaters, Hoopty Lawn Care on the doors and start trolling the San Fernando Valley for Boxsters.
Buick Grand National
No matter how you slice it, the Grand National looks like a Buick Regal. "But I have seen Grand Nationals that are absolutely SICK!" Yeah, me too. I have seen amazing Vegas and Pintos too. But it still looks like a Regal. Another prime candidate for badge removal.
Mercedes 300 SEL 6.3
The herringbone interior is really such a quintessentially German thing to do. The interior looks like Howard Cosell's sportjacket at an insurance convention. To this day, one of the scariest rides of my life was in my good friend Chad's 6.3 as he was trying to point out all the performance aspects of the self leveling suspension. I swear we were on two wheels on the freeway exit ramp. Annual maintenance on these cars is something akin to the GDP of Antigua.