Favorite G-Body, eh?
As for exterior looks, well, that won't be too difficult for me: the '84+ Regal easily wins. (This is all the fault of the GN, of course. I remember in high school when I'd be out cruising around on weekend nights and I'd pull into the local Buick dealership just so I could walk their lot and lust over the then-new '86 and '87 GNs. I've wanted one ever since then, and I've still never owned one... and I probably never will.) I like the '84+ Regal's "rounded off" nose much more than the more pointy '81-'83 design, and I also prefer the "segmented" tail lights that debuted in '84 as well. The biggest downside to the Regal, as we all know, is the horrid instrument panel design. Yes, I know they were generally marketing the Regal to doctors and such... but why couldn't GM be bothered to offer a full set of gauges in this car like they did with the other three??? (Yes, I know about the GNX... but we are talking about the 'normal' models here.)
As far as the Monte Carlo goes, the NASCAR nose on the SS looks fantastic. If the '84+ Regal didn't exist, that would be my favorite exterior design. In non-SS form, though, I don't really care for the Monte--the nose is just too flat for my tastes. However, I'd still rank it higher than the pre-'84 Regal since the Monte at least offered a full gauge package. For similar reasons, I've never really cared much for the El Camino, though I certainly respect those as far as their utility is concerned.
The '81+ Cutlass looks absolutely fine. I think the Euro headlights add the final touch that really 'cleans up' the nose, but even with the usual quad headlights they look fine to me.
I always felt the Grand Prix was the ugly duckling of the family. (Ironic, I know, considering what I now own.) The flat nose with the overly-prominent chrome grille was just too... garish for me. Strangely enough, the Pontiac is the only one where I prefer the pre-facelift look; I love the parking lights between the headlights that you didn't get on any other G-body. On the plus side, the GP has far and away the best dashboard design to me! All those nifty little circular holes for each gauge just look fantastic. For this reason, I'd have to put the GP above the regular Monte Carlo overall.
In summary, here is my list of favorite G-Bodies in order:
1. '84+ Regal (especially the GN)
2. '83+ Monte Carlo SS
3. '81+ Cutlass
4. '78-'80 Grand Prix
5. '81+ Grand Prix
6. (tie) non-SS '81+ Monte Carlo & '78-'80 Cutlass
7. '81-'83 Regal (as mentioned, I don't care for the protruding grille)
8. '78-'80 Regal (this grille protrudes even more)
9. '78-'80 Monte Carlo (definitely not a fan of the forward-slanted front fenders)
(Yes, I realize I left the LeMans, Grand Am, Malibu, and Century off the list. Sorry.)