Not having been there, I can't speak for the 60s, 70s, or several of the decades that followed, but here's how I see each of these words as of the present:
....is mildly insulting, even less so as a verb (I am in the US; it is apparently more offensive in the UK). NOAD doesn’t even mention it being offensive at all. Cerebral palsy is no longer described with "spaz", so the connection between them is not obvious (I didn't know about it til I looked it up). "Spazz" is also part of the names of several stores/products I've seen, and although they're unrelated etymologically, this also serves to weaken the offensiveness of the word (or at least shows how weak it is).
...has also been used an insult as long as I can remember. Although its other forms ("retarded", "retardation", and the verb "retard") range from sometimes not insulting to never insulting, "retard" is basically always an insult, and a pretty strong one I think too. Clinically, "intellectually disabled" (ID) is used sometimes instead of "mentally retarded" (MR). It may have originated decades ago as an insult, but it still has currency as one.
...in contrast to the other two words, is mostly used inoffensively to describe the developmental disorder. It's a different part of speech than the other two. There's also no good substitute (a la euphemism treadmill) to refer to the condition either ("Asperger's": too specific, "ASD": contains the word "autism", "on the spectrum": verbose). It's a little hard to judge, but certainly "autistic" is used less as an insult versus "retard" (or "retarded"), as can be seen in Corpus of Global Web-Based English (GloWbE), which has a mix of news articles, comments, and forum posts from the web. It's unfortunately more common in (online) gaming, as this post about League of Legends (a game notorious for having toxic chat) indicates.
I don't remember it always being an insult, but it is definitely an insult now. For me, it's the strongest of the three words, although I may be biased.
Oh, and I guess it's also relevant to note that Asperger's (and its derivatives) is also used pejoratively sometimes. The most common I've heard in this respect is "sperg out".
All in all, I would say that both "retard" and "autistic" are both very current insults.