I have to say the 455s (early versions) and 317s (ditto). The later versions of both had the cab roof line smoothed, making a huge difference. The various 'Pacers' were pretty dire as well.
The aesthetic side of train design is, understandably, often placed low on the scale of importance, but some iconic designs have emerged almost purely based on looks - the A4s with their original wheel valances were largely a result of the importance attached to design in the 20s and 30s (and only partly due to genuine aerodynamics which really only kick in with any significance above about 90/100mph ), and the Bulleid pacifics' air-smoothing was really just a design statement which made maintenance harder! I remember my father telling me that the first time he saw a brand new 'Merchant Navy' at Woking in 1941, he thought it was science fiction, it was such a radical design! The opposite applied to another Bulleid design, the Q1 0-6-0, which was built to be as easy to maintain as possible and looked like a badly-built kit as a result!
Slight deviation from the EMU/DMU subject but the same principles apply - concentration on form or function often produces different results. It's often the case that functional designs arise in times of austerity - hence the cheap and cheerful/miserable 'Pacers' were attempts to provide low-cost rail vehicles at a time when the railways were struggling to be cost-effective.
BTW, I always liked the original 309s with wrap-round windscreens and in BR maroon livery - very elegant and fast (100mph) units.
