I've always been fond of:
Druid
Cleric or Archivist
Beguiler
Conjurer
None of these gets pinned down into any one role. All are full casters. The group can adapt pretty well in any given combat, and remarkably well given a day's notice.
Seconded. You don't need a meatshield because A.) you have a druid and B.) your conjurer can create his own meatshields. Trapfinding and face-duty is taken care of by the beguiler. You have enough skill points (and varying class skills) between the four of you to do pretty much whatever the hell you want. The Druid handles survival and nature-based skills. The Conjurer does all the knowledge-monkeying. The Cleric does whatever his domains encourage him to do, or becomes another knowledge-monkey if he's an Archivist (you can never have too many knowledge monkeys). The Beguiler does everything else.
SR should pose no problem, since the Conjurer's packing a metric crapton of SR: No spells. Three outta four are capable summoners - the cleric should concentrate on building up an army of walking dead, the druid brings in a pissed-off animal platoon, while the Conjurer bends planar denizens to his will.
Of course, if you want me to answer the ACTUAL question, it comes down to:
Factotum (most flexible base class in the game)
Binder (second-most flexible)
Bard (hey, jacks-of-all-trades are known for their adaptability)
Artificer (arguably not dependent on high-level spellcasting).
They all have tons of skills, at least two have permanent trapfinding (while another can get it). Each one has some spellcasting (nothing that
really breaks the game, though, except for possibly the Artificer) and decent combat ability (all having at least medium progression). You could easily replace any of them with a Beguiler, since they're not exactly a tier 1 caster to begin with and are incredibly versatile as well, but they do rely on their spells and so I did not include them in the primary list.