I've not personally driven a car in the same class I preferred. The C5 is close on the drive, isolation and comfort (if anything better on the cars with Hydractive 3+ suspension, although if the 'plain jane' Hydractive 3 on the 2.0 HDi rolls as much in the corners as my mate's Xantia, you think you're going to fall off the road.)
IMO, the contemporary facelifted Vectra-B and Mondeo Mk3 both feel 'tinny' with much higher cabin noise levels, the Mondeo handles nicely but is a bit too stiff for the roads round here (lots of potholes... and I've got Silverstones now
), even in non-sporty trim. The Vectra-B handles like... Well, it doesn't really bother, in actual fact. Also, I noticed the boot lip on the hatch is irritatingly high, and the estate is tiny
I've never had any experience driving any of the Japanese equivalents, although the Accord seems to constantly win awards. From what I can recall of my dad's old one, I'm not a massive fan of the B5.5 Passat to drive (understeer extreme?!) but I do find it has more of a sense of isolation and is quieter than most of the others (I guess that contributes to the 'luxury car' feel..)
I certainly think the Lag 2 is the most 'balanced' car to drive of the lot and probably the most pleasant to live with if they were just a smidge more reliable - I know the steering is a little bit uncommunicative and the self-centring action is a bit heavy, but that's part of the reason it's so relaxing to drive, it certainly still holds on in the corners well though!