Please accept these are only my opinions, if anybody agrees with me then I’ll be amazed - a variation on a theme by the OP.
Atomic Rooster started wonderfully well with an interesting debut followed by two great albums an then what happened … Vince threw out John Du Cann. Seemed crazy - oh but then it was Vincent Crane.
The Beatles - agree with OP and they were the pinnacle of popular music for me. Sgt Pepper and Abbey Road are as it good as it gets in a studio.
Black Sabbath weren’t bad - I liked Master of Reality
Blossom Toes who remembers them? Two great albums, but nothing more. We Are Ever So Clean is almost as good as Sgt Pepper.
Curved Air - peaked at the outset and produced 3 very good albums after which the albums were merely good. You can’t do much wrong with Sonja Kristina in the lead.
Deep Purple Mk2 hit with a bang at the beginning with Concert for Group and Orchestra, but for me their best albums were The Book of Taliesyn (Mk 1) and Machine Head (Mk 2).
Emerson, Lake & Palmer maintained very high standards across the first 5 albums. It definitely dropped off afterwards though each of the subsequent albums has something to offer. Of course, Brain Salad Surgery is a masterpiece and even better than Tarkus. Keith’s solo albums are wonderful too especially Emerson Plays Emerson and his autobiography hilarious. His death was tragic.
Fairport Convention - it has to be the trio of albums in 1969 and that means Sandy Denny. Probably the greatest song writer of my life time and irreplaceable. Her solo albums are wonderful too with Sandy being her pinnacle. Of course, Richard Thompson was there too so no wonder it was their peak.
Genesis hit their peak with Trespass with The Knife as their best ever track. Anthony Phillips continues to make superb records with no sense of decline.
Hawkwind still great - their opening albums were excellent, but their peak was when Robert Calvert took on song craft duties. The peak for me was Quarks, Strangeness & Charm with Calvert it was X in Search of Space with Dave Brock at his brilliant best
Incredible String Band - the first three albums are their best and A Very Cellular Song is their peak of creativity though when it comes to albums I prefer 5000 Spirits to Hangman’s Beautiful Daughter. ISB backed Shirley Collins on The Power of the True Love Knot and that is simply superb. Shirley went on to record Anthems in Eden and it remains at the top of my list of great albums.
King Crimson have what I hope Coronavirus hasn’t - lots of peaks. Perhaps their debut is their best, but then there is Lark’s Tounges and there’s Three of a Perfect Pair. Wonderful band(s).
The Kinks - it is their early material that we all love or should do - everything up until Lola vs the Powerman on the Moneygoround. Ray Davies’ greatest song from so many great songs was probably Dead End Street.
Led Zeppelin hit the heights with their 3rd and 4th albums. The Battle of Evermore has Sandy Denny sharing lead with Robert Plant so will forever be my favourite Zep track.
The Moody Blues - Go Now apart it was a shaky start, but then Days of Future Passed happened and album after album flowed. So many great songs from Nights in White Satin to I’m Just A Singer in a Rock & Roll Band. Later albums like December are embarrassing on my view.
Move/ELO - Roy Wood has always entertained, but his work with the Move has always stood out and albums like Shazam and singles like Fire Brigade and Blackberry Way score higher for me than Wizzard. Jeff Lynne is another star performer, but it was early on that ELO set the bar for rock music with Eldorado. It’s never been bettered.
Nirvana had the good sense to call it day after a couple of albums, but The Story of Simon Simopath has only one fault, it is too short.
Ozric Tentacles - simply wonderful and still producing the goods, but for me Erpland early in their career was the pinnacle.
The Pink Floyd started magnificinantly with The Piper at the Gates of Dawn and 2 of the best singles ever recorded then Syd wasn’t well and went back to the hotel. Syd’s solo albums The Madcap Laughs and Barrett are almost as good, but then …
The Pretty Things - great band, but the stand out is SF Sorrow so good it could have been the Beatles. Defecting Grey is a wonderful single so good it could have been Syd. The follow up Parachute was very good, but all downhill after that.
The Rolling Stones did record Their Santanic Majesty’s Request and some excellent singles before Brian Jones died.
The Small Faces peaked at the end with Ogden’s But Gone Flake. Not that the early records were bad. When Rod Stewart replaced Steve Marriott then the decline had started even if it meant they weren’t small anymore. Steve went in to decline too after a great start with Humble Pie.
Van Der Graaf Generator - no they didn’t peak, they stayed on a plateau that was higher than most croups ever attain - and solo albums by the wonderful Peter Hammill likewise. And founder member, Chris Judge Smith continues to record great albums. So they rose, but never fell. Though I doubt VDG will record another album.
The Who started well and the sold out. Sorry I mean peaked with The Who Sell Out
Wimple Winch - they made that single, my favourite 45 rpm 7 inch masterpiece: Save My Soul - gone and forgotten, but what a great record. A few other good tracks, but not enough for even one album.
Zombies - started well with She’s Not There and ended with one of the great albums Odessey and Oracle - they couldn’t spell, but they could write great songs. Yes they morphed in to the very fine Argent and the addition of Russ Ballard was a great move as in evidence on Ring of Hands
So my conclusion is most. but not all bands are better when they first emerge.