Having owned and used all of them over the span of many years, here are my oppinions regarding Classic AMIGA case styling. These oppinions are purely related to cosmetic styling and in no way indicate my oppinions in regards to functionality, unless specifically stated.
WEDGES:
The A500 is the best looking "wedge". It's AERODYNAMIC, man! It's the only one that could compete with an ATARI 520/1040ST in a "wind-tunnel test". hahah. (Thats about the ONLY thing that the ST had going for it. Oh wait. I better mention the Built in MIDI ports so I don't piss off the ATARI people.) But seriously, these machines look particularly kewl with a GVP (or ICD Trifecta) sidecar expansion attatched.
The A600 looks like an Apple IIc... and the A1200 looks like a wide Apple IIc. (Although I would still vastly prefer the original cases to any "PC-looking" Tower case).
DESKTOPS:
The A3000 desktop is the coolest looking "big box". It has got the look that would have been "right at home" in a room full of SparcStations and SGI workstations.
The A1000 would come in second place.. These machines have that whole "early pizza-box style workstation" look to them. The nonstandard keyboard is not too appealing, but it does actually function well and its kinda kewl that it slides under the case perfectly. This sytem is also REALLY close to 19 inches wide. Ive often thought these would be great for mounting in a music/equipment rack.
The A4000 desktop is a definite improvement over the A2000 case and definitely has what I would consider "distinctive" AMIGA stlying, but lacks the sleek profile of the A3000 desktop and is approaching the realms of the casework used in some AT clone desktops of the day.
The A2000 has by far (in my oppinion) the least appealing cosmetics of any AMIGA made by commodore.(Not to say that we all wouldn't have KILLED for one back in 1988/89,) but there were plenty of XT (and early AT) desktop clones that had nicer ("cooler/sexier") looking cases...
TOWERS
The A4000T looks like a PC tower from the 486-Pentium III era. It's looks were "trendy" and "respectable" for it's time, but not exactly unique, and certainly not "distinctive" as an AMIGA.
The A3000T is unique, and is an awesome machine, but I don't know if Id classify it as "pretty". Having owned one, I am partial to these machines, but I could see how the average person might mistake it for some kind of mid-80s DEC or IBM mini-computer..
ENTERTAINMENT CONSOLES
The CDTV looks like a VCR or early Home-Audio CD-PLAYER component.
The CD32 looks similar to a whole host of machines made in the CD-based game console era.
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Well, there it is. These are my personal oppinions as a long-time AMIGA owner/user. I know you didn't ask for a "research paper" but I felt like I should explain/elaborate. The average person's views may very well be completely different.