Great idea for a thread Hombre!
Your three examples:
Zombies - for me they could never convincingly pull off that chirpy, catchy, happy-go-lucky style (a la Herman's Hermits) nor the high octane, adrenaline pumping, party stompalongs (a la DC5) which the sixties pop pickers loved. More talented perhaps. But so much LESS charismatic and "entertaining" in a showbiz sense. Not being mean but the singles-buying masses just didn't go in for the subtler stuff back in '65. What The Zombies, like The Yardbirds, did well was the sort of slightly darker, more sombre seasoning. "She's Not There" and "Heart Full Of Soul" are brilliant but bordering on morbid and I think Joe Public could only stomach small doses of those kind of records in those days.
Doors - I think you are 100% correct - ahead of its time as a singles release and like you I believe it would have been a stormer had they unveiled it in '68. Certainly time has been good to it and it deserves its place on those "Best Of" compilations irrespective of chart placing. There is a myth that everything and everyone was hip and cool by '67. Not so! Even The Beatles couldn't push their cutting edge "Strawberry Fields Forever" past Engelbert!!!
Love - I've said before that I don't "get" the kudos of "Forever Changes" - but I've always felt "Alone Again Or" is a very good track. Why not a hit? Like The Doors in my opinion - a bit ahead of its time but also too "studenty" (a bit too serious and full of itself). It didn't need much tweaking, just a small injection of fun...but of course, that would have been cringeworthy and uncool. Nevertheless, by keeping those lovely acoustic guitars and the brassy orchestral middle - but pepping up the ascending vocals and adding a gimmicky bullwhip, Dave Dee, Dozy, Beaky, Mick & Titch brought us "The Legend Of Xanadu" and hey presto! Number one hit!
I will now have to go away and think of some examples of my own!