Going against the grain somewhat, but in my opinion, the single seater usually looks better! Especially true in the case of the Typhoon and F-16 - both look ungainly and somewhat humpbacked in twin stick form, especially the Tiffy. Two seat Harriers as well, all ugly as sin - appealing and interesting regardless, but bloody ungainly all the same!
Single Seater looks best:
Typhoon
F-16
Viggen
F-15
Any Harrier
Anything Russian (see MiG-25U for an interesting looking if bloody odd machine)
Classical Brit stuff (Hunter, Lightning, abominations looks wise in twin stick form IMHO compared to the sleek and brutal single seaters respectively)
Twin seater looks best:
Gripen
Jag
Hawk (Hawk 200 looks very awkward compared to the smooth lines of the pre-100 series Hawks)
However whilst I say this, the twin seat ones I've listed as being ugly are still subjects I'd like to model - ugly can be appealing when it comes to (plastic) models, see the A-10 and Apache as proof