I was going to answer the question with "TM Bill for sure", but then I realised it was about bikes...
It's actually a really hard question to answer - but on the bright side there's no right or wrong answers, just opinions. Allow me to crap on for a bit while I think about it...
To my eyes, all pre-70 bikes look kinda clunky. So why not go all out for the clunky-ness and go for the leading link Greeves? I recognise that they're not a 'pretty' bike, but I like the way they look, largely
because that they didn't try to be anything other than functional. The RH67 is cool too - half of it is incredibly cobby looking, the other half really quite nice. I don't mind Glenn W's ('68?) Husky either - not so cobbly.
Pre-75 is easy for me: 1971 YZ250. I also like the Ossa Phantom for the same reasons I like the RH67. But I don't think any of the pre-75 MX bikes are ugly - even relative 'dross' like MX250s are pretty good looking bikes in my eyes.
Pre-78: '77 RM125.
In the world of Evo bikes, I really like the all white Huskies and the CR250RZ. But the (Italian) TM125 is the winner.
Pre-85: 83 or 84 YZ125/250 are the best of an un-inspiring bunch. The Cagivas (like in the recent VMX) are good, but I'll reserve judgement until I see one in the flesh.
Later stuff:
YZM500.
Leisk-era HRC500s.
In the world of production bikes, the 87 and 88 Honda CRs get my vote.
I also reckon the 91~93 Huskys are some of the best looking bikes of the last 20 years.
Never seen a TM that I don't think is good looking, and the early GasGases deserve a mention.
Hmm, now that I've thought about it, the 71 YZ250 works bike gets my vote.
