See if this one has legs - nominations for cars which were worse than their predecessors - so instead of being The Great Leap Forwards which their marketing people claimed, the reality was A Great Leap Backwards. Suggest we try not to judge only on styling - that would be just too easy. Anyway, to start things rolling:
- any Peugeot after the 406,
- Golf Mk3,
- Jag XJS,
- the first Vauxhall Vectra/2nd Opel Vectra,
- the second Vauxhall Vectra/3rd Opel Vectra,
- Merc S-Klasse V140,
- Fiat 132,
- Ferrari 348,
- all 1970s small/medium Datsuns apart from first Cherry,
and so on.

Ford Escort MKV
Jaguar XJ40
Peugeot 307

the current Renault Twingo (even if as a car it is very good, it is not a real replacement of the Twingo)

Allegro
Triumph TR7
Toyota Avensis (not to say that the Carina was any good)
2010 Subaru Legacy
Cortina Mk3
Ford Sierra
Range Rover P38
Fiat Stilo
Citroen ZX
BMW X5 [E70]
BMW 6 [E64] - I'd like to nominate all Bangled BMWs because they're so badly styled, but unfortunately most of them are good cars underneath.

Any VAZ after 1993.

Vauxhall Signum (As a replacement for the Omega.)
Ford Ka MKII (... And the first one was utter shit.)
Ford Mustang II

I sort of liked the Signum, though never drove one. Supposed to be a used car bargain if you can find one. And the 1st Ka was so bad, I can't imagine the Ka 2 was worse.

There was nothing particularly wrong with the Signum (we had one as a hire car once and it had a lot of space and all the strengths of the Vectra) but the idea was deeply flawed.

They managed to turn their backs on the main customer base of the Omega (the police) and also tried to compete in the executive market with a car whose Vectra origins were obvious. It had the double whammy of being a hatchback and a Vauxhall which meant it struggled to sell to image concious buyers. It might have done well if they priced it closer to the Vectra but they charged a large premium.

One important example is missing:
Ford Scorpio I (worse than the Granada)
Scorpio II (worse than all)

- Mercedes W210 (worse, much more unrelieable than the W 124)
- W124 (same situation 11 years before, compared with the W123)

- all Benzes after 1991, as the fans are saying. Some say after 1993

- all Opels from the Lopez-times, at least all from the times, when US-americans were the management

- all VW's from the Lopez-times

- VW T4

About Ford: in a classic magazine was written once, that the best and most reliable fords had been in the T- and the A-Model, the V8, too. In Germany the 1939/41, resp.1948 "Buckeltaunus", too - but everything following was worst. The first trace of reliability came back again with the first Focus (resp.the design ist terrible disgusting).

About the Signum: it's nothing else than a Vectra C with a cutted back. The whole car up to the C-pillar is identical.
And the Omega has indeed much more space than them. The Opel-salesman has confirmed that, when I made a test-seating some weeks ago. He told me, that some customers, which still have Omegas, want to keep them longer, becaue the modern Opels are so narrow inside.
This is true, as my own experience of the last week has confirmed

For U.S. vehicles:

The 1996 Ford Taurus that debuted for 1995.

I would like to nominate the future 2013 Jeep Grand Wagoneer. Sure it’s not fair to judge a vehicle which is not even in production yet, but no matter how good it is (if it is any good), it will never be what the original Wagoneer was. Those will be some tough shoes to fill.

The 1988 GM full-size pickups, and the 1992 GM full-size utilities that followed soon thereafter. The interior was simply horrendous. Cheap hard plastic everywhere, combined with trim & panel gaps that could sometimes be mistaken for the Grand Canyon.

They are durable, reliable workhorses, but if you seek a workhorse with quality and refinement, look elsewhere, such as Ford’s pickups from the same era. Stay away from GM.

As for Grand Wagoneer...It's clearley another trying to revive once great name, because they can't offer anything better. It's all about nostalgia...but it can't work all times. By their attempts to recreate old glorious names and imitate their styling they just prove theirself unable to just make a good car, which would be competitive.

It doesn't really count as a successor but putting the DS name on that grotesque DS4... I don't mind the Ds3 that much (Aside from the horribly bright LED DRLs that blind you if one is driving in your direction) but none of these cars have the innovation and the sheer radicalness of the original DS.

Mini 1275GT (compared to Cooper S)
Ferrari 365 BB (compared to Daytona)
Ferrari 512 Testa Rossa (bloated whale compared to BB)
1968 Austin 3-litre (the Westminsters were well past their sell-by date, but....)
every Subaru Impreza since 2001
Mk4 Zephyr/Zodiacs - the only British attempt to create land yachts
VW 411
Focus Mk3
Alfa 33
Fiat 126
1974 Porsche 911 Carrera
all Porsche 911 [996].

FWD Volvos

Err, dsl, the example VW 411 doesn't fit here. It had no predecessor and no successor (as the K 70, too). The Passat is the successor of the 1600 Typ 3. The competitor in the own house (after 1969), the Audi 100 C1, doesn't really fit here either.