Diazepam kind of sucks, tbh. Clonazepam—whether the intended use is as a skeletal muscle relaxant, sedative or hypnotic, anxiolytic, CNS depressant, or even as a recreational drug—outstrips diazepam by a substantive margin, at least in my experienced opinion. I never got the appeal of diazepam, and the comparative unpopularity of clonazepam is even less understandable.
So, I'd be dishonest if I told you diazepam was the better muscle relaxant of the two.
^Ritalin can be fun, yes. I find however that at high dosages it is not nearly as smooth as mixed amphetamine salts (Adderall)...Ritalin feels rather dirty. However 10 - 20 mg is great for social settings or just about anything (you will feel mildly on top of the world).
Now...take 40 or 50 mg of mixed amphetamine salts (IR) you will feel like superman, but don't take 50 mgs of Ritalin IR without expecting some nasty side effects...though I must mention there will be quite a bad comedown from 50 mg Adderall IR also...just not as bad as with Ritalin.
IMO Ritalin has less abuse potential than amphetamine.
[While this topic is not at all germane or relevant to the OP, I still feel like briefly commenting on it.]
I agree about methylphenidate having a lesser abuse potential than amphetamine (especially dextroamphetamine). I find it difficult to imagine myself being able to understand how or why anyone could sincerely believe the obverse is the case.
However, having had years of experience with both Ritalin and Adderall, I would disagree about the latter having a 'smoother' or 'less dirty' high and a relatively less uncomfortable comedown than the former. The turbulence one encounters on the descent from a high dose (~80mg or so, in my opinion) of Adderall is, so to say, formidable.
I've actually taken around 150mg of (d)amphetamine today so far. My last dose was 40mg, and was taken about 2 hours ago. I don't feel 'smooth', to say the least. In fact, my eyes feel uncomfortably strained and anhydrous, my extremities are annoyingly tremulous and feel exhausted and weak, the interior of my mouth feels dry and desiccated as if I'd been destitute of water for days or had invested the last 6 hours in eating spoonfuls of salt, my movements have become lurched and stereotyped like a malfunctioning poorly-constructed android, my actions or activities have become uncoordinated and discursive, my thoughts are increasingly nervous and tweaker-esque, and so forth.
Notwithstanding all that, I could probably find your opinion agreeable if we are to limit the maximum daily amount of amphetamine to 50mg total. Perhaps such a seemingly paltry day's dose would have smoother effects than x amount of methylphenidate. But while that may be, I would still say that, after a certain point has been passed at the upper dosage levels, amphetamine becomes patently more unsmooth in effects than methylphenidate.
But I'm far too intemperate with my drug use to ever stop at only 50mg of amphetamine. I average about 180-240 mg per day, usually taking the drug in 60mg amounts every 4 hours, from the moment I awake until the moment I prepare myself for sleep (with my routine bedtime preparation, consisting of a handful or two of downers and about 350ml vodka with which to wash 'em all down, of course).