In my opinion, based on cars available in the U.S:
Most fail-tastic subcompact:
Chevrolet Aveo - because stupid people buy them because it's a "real American compact" without realizing that it's actually a Daewoo with some Chevy badges plastered on. Build quality is horrible and, based on my time working at a GM dealership, they are horrendously unreliable. It's on the cheap side, but is that really worth it considering that I've seen a few being pushed back to the service department with less than a thousand miles?
The Pontiac G3 is even worse, if only because of the badge-whoring. Driving excitement my ass.
Most fail-tastic compact:
Chrysler PT Cruiser - because it had so much potential but it was left to rot on the vine. Another vehicle that I have seen with countless quality issues as well. Could have continued thriving but Chrysler insisted instead on making special editions rather than actually updating the car. These include the Touring Signature Series, Street Cruiser, Walter P. Edition, Limited Edition Platinum Series, and Dream Cruiser.
However, it does have a nifty shift knob.
Most fail-tastic mid-size sedan:
Chrysler Sebring - Sure, the latest generation does not have the Handgrenade V6, but it's still a terrible car. Those stupid ribs on the hood don't help either. Everything's plastic and everything's dull and cheap. It sin't even well designed - the battery is located in the wheel well for Christ sake. Reliability issues abound and terrible fuel mileage.
Most fail-tastic SUV/Crossover:
ANY L-Platform GM product - Yes, that's the GMC Acadia, Buick Enclave and Chevrolet Traverse. The Saturn Outlook is also included but that one was literally an Acadia with Saturn badges. Consumer satisfaction on all of these are below average, despite GM's advertisements stating otherwise. L-Platform GM crossovers do absolutely nothing right.
They aren't well built. I used to work at a GM dealership and every day (I stress that bit) there was at least one Acadia with its entire interior torn out. The problem? Either squeaks and rattles or severe water leaks. The interiors are cheap in appearance and the fit and finish of everything is what we have come to expect from GM.
They are not reliable. Nobody likes a brand new car that they just bought to start taking on water or simply shatter a driveline component. One customer reports taking his to service fifteen times since he bought it, and it doesn't even have 30,000 miles. That wouldn't even be fifteen oil changes on a reliable car.
They are not easy to service. Even something as simple as an oil change is a hassle. The oil filter is a regular screw-on type, but mounted on the very front of the engine, but down adjacent to the exhaust manifold. You cannot get a strap wrench on it without burning your wrist. Mind you, that's simply the oil filter. Try servicing much else on the Acadia and you will question your life path as a service technician. I know I sure did. The L-Platform is one of the reasons why I stopped working for GM.
I could go on for days. Basically, the Acadia/Traverse/Enclave are for people who would like an awkwardly-styled SUV that is not useful off road, not terribly reliable, not very economical, not cheap nor well built, not fun or interesting to drive, not very comfortable or innovative. If that's everything you could want and more, wait for the 2011 models to arrive because they are now going to produce a GMC Acadia Denali* for an even higher price and douchebag factor.
* Denali - Pronounced "DEN-AL-EE" by normal people. Pronounced "DIN-EE-LEE" by Denali owners.
Most Disappointing Car of the Decade:
Buick Rendezvous
Oh god, please don't get me started on this miserable pile of under-engineered garbage. It's an even more awkward looking Buick SUV that's riding on a minivan chassis and has absolutely no redeeming qualities whatsoever. Take all of the problems with the L-Platform and just toss them right in with all of the issues that plague this unreliable, disgusting POS.
Who the hell actually buys a Buick Rendezvous? I ask myself that question every time I see one on the road. Its natural habitat, however, is the service bay. I really like how the Rendezvous starts eating antifreeze for no apparent reason, typically just after the warranty expires. One particular Rendezvous just started blowing modules, again for no explainable reason. It required three new BCMs in under 12,000 miles. No technician could find the actual reason it was doing this.
I have to stop this now. Talking about the Rendezvous makes me want to go punch something really hard.
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Edit: Although this is a little bit off the decade, it's still a big enough disappointment to be featured here.
Most fail-tastic "cool" car:
Plymouth Prowler (Yes, I know it started production in '97)
Ugh, if this was not the biggest automotive slap in the face in the history of automobiles, I don't know what is. I remember when this was coming out. It seemed like it was going to be so incredible and awesome, and cool. Instead, we got a half-baked roadster with the same underpinnings as a Dodge Intrepid. Despite the hot-rod pretenses, the car was only offered with an automatic gearbox and an underpowered V6 engine. Mercifully, production ended eventually and it was replaced with the Chrysler Crossfire. Oh joy!
Fun fact: In 1998 a Plymouth Prowler was sealed in a mausoleum as a time capsule in Tulsa, Oklahoma. Why? In 2048, the car is going to be unveiled and show the citizens what exactly? Something that Americans from the past looked at as a symbol of their past?