IT's like saying "The Beatles were the greatest rock group ever; that's a fact." Actually, no, it isn't.
Look, I understand where you're coming from. I hated the Dead A LOT at first. Friends would thrust tape after tape of the Dead at me, hoping I'd like it, and I didn't. But after a while, i began hearing things in the jams that would make my ears prick up; it helped that most of the tapes my friends would give me were pretty lo-fi, because it made the music sound alien, otherworldly. in fact, For me it was a small step from hearing the Dead on skeevy audience tapes to listening to Caroliner and Guided by Voices's lo-fi shit with the same mindset (i.e truly messed-up improv rock damage from another planet). It also helped that I had already heard Sun Ra and other free-jazzers, and could incorporate dissonance and noise into my listening spectrum without displeasure.
Anyhow, i know why people dislike the Dead. In fact, i STILL can't stand it when they try to play the blues (ugh!) or Chuck Berry (UGH!!). They fucked around a lot and embarrassed themselves even more. However, the part about them being amateurs is problematic. Garcia was probably the most accopmlished banjo player on the West Coast before he even began playing electric guitar in a rock idiom (pre-1964), for instance, and Phil Lesh was a facile composer studying at San Mateo college (under Luciano Berio), and only picked up the bass AFTER garcia hired him for the band. Garcia had an encyclopedic knowledge of folk music. Seeing "A Hard Day's Night" gave them the impetus to begin playing rock music on instruments some of them hadn't played before. Yes, they had limited ability - that wasn't the point. They point was that the Dead, even in their earliest incarnation, were carving out a new thing entirely: collective improvisation within the rock idiom, influenced by the formlessness and chance of the LSD experience. Sometimes, it sounded like shit. Other times, when hit by a gust of inspiration, it could be semi-magical/religious (in the opinion of many). The Dead learned on the job, and that's why their music sounds nothing like any other band's. That's also why the uninitiated often think it sounds like a bunch of drunken amateurs only occasionally hitting the right note. What they were actually doing was holding conversations with one another on stage through their instruments, and thus the improvisations were shaped by the vagaries of their emotions, their abilities, and the drugs they were on. Self-indugent, yes, but if you're fascinated by the idea of COLLECTIVE improvisation (i.e. not just a soloist with backing, but a group of individuals coalescing into something greater than the sum of their parts), then the Dead's jams are pretty enlightening. There's times when Lesh and Kreutzmann lock up so tightly (the "Playin' in the Band" jam from 2/9/73 comes to mind (you can listen to it on archive.org here:
http://www.archive.org/details/gd1973-02-09.sbd.ashley.12571.shnf)) that you'd SWEAR they'd rehearsed it. But in order to get to that sweet little nibble, you have to wade through a lot of out-of-tune singing, blown chords, fucked-up signals, and sloppy nonsense. In that context, you could either give up listening, or you could claim that the surrounding nonsense gives the nibble value and meaning. I prefer the latter option. YMMV, of course. Kinda like wading through sewage and finding a ruby.
Oh, yeah, and there's also the issue of timbre. The Dead were a prickly bunch who refused to make their instruments sound like any other musician's. Some eras are unlistenable to my ears because of this intransigence: 1994-5, for instance, or a ot of 1983-84. Those were eras when it seems like the tonal qualities of their guitars, hi-hats, toms, and synths all made for some awfully unpleasant shows. But there's also an admirable aspect to that stubbornness as well - it's not like they were doing something mega-commercial, ya know. It was actually that stubbornness that inspired Deadheads like Greg Ginn (of Black Flag) and Lee Ranaldo (of Sonic Youth) to explore, define, and weaponize their own unusual tonal qualities withing their own band-voice contexts.
Sure, they played SLOWER and sloppier than many other bands, but there were periods (especially November 1972 thru March 1973) that they could have duked it out with any of Miles Davis's electric bands whenever they entered a jam. They were at the top of their game in that era, tight as shit and nimble too. But, indeed, even then they insisted on a slack aesthetic that ensured that mistakes were not only made but encouraged (mistakes often lead to happy accidents, as Bob Ross might say). However, they had control over what they were doing musically, even if they never played with the lightning speed or precision of Mahavishnu or whoever. They were never "great musicians" in the traditional sense. They CREATED the realm and the criteria in which they excelled.
Back to the "amateur" thing: now that i think about it, being "amateurs" isn't always a bad thing. If it hadn't been for amateurs, there'd never be punk rock. I know, through anecdotal evidence, that seeing the Dead fuck up on stage was an inspirational thing for lots of musicians who would have otherwise been too timid to play in front of others. Like Trey Anastasio once said of Pavement, their beauty is in their willingness to suck. The Dead, in a way, were doing something that was sorta punk rock - they didn't give a fuck about selling records, they played out of tune, they knew they couldn't hit all the notes and they did it anyway, etc. Kinda like an American Hawkwind, I guess.
Okay, it's getting late. I can understand COMPLETELY why you think they suck. but simply insisting that your dislike of them is not opinion but "fact" is nothing but trying to incite people, to ruffle feathers, and it's transparent. It's much easier and much more faithful to the tenets of reality to just say "I really hate them, and here's why..." than to say, "They are terrible, and that's a fact." Because it causes fewer flamewars and fewer people will think you're an asshole. However, if your intention was to provoke people into thinking you're an asshole, so be it. That's fine, too.