I hear that the Small Faces were criminally underrated in the US, at least in the Sixties. Thankfully there seems to be a bit of a reassesment of them underway. Truly one of the best mod/psych bands the UK had to offer- and they only lasted around 5 years from 1964-69.
Anyway, they're not really forgotten as such. Here's a few that I feel are:
The Chocolate Watchband
Spencer Davis Group (I mean after Steve Winwood left to form Traffic in '67)
The Master's Apprentices (Big in Australia but not so much elsewhere)
The Misunderstood (The late John Peel cited 'I Can Take You To The Sun' as one of the all-time greatest records, and who am I to disagree?)
The Monkees (Not so much forgotten as underrated. A 60's pop band on par with the Hollies in my estimation.)
John's Children/Tyrranosaurus Rex- both featured Marc Bolan, whilst the latter would eventually evolve into 'T.Rex' of glam rock fame.
Grapefruit- the first band signed to Apple Records. Disappeared amidst massive speculation and expectations which were impossible for the band to live up to.
Warm Sounds- As the name suggests, torch holders for the sunny, sunshine pop sound. This English duo scored a pirate radio number one with 'Birds & Bees' in '67, which also reached #27 in the Nationals.
Procol Harum- Everyone remembers the era-defining hit 'A Whiter Shade Of Pale'. However, people are much to quick to dismiss Procol Harum as a one hit wonder- the follow up single 'Homburg' reached #5 here in the UK, whilst they also released a number of truly unique but incredibly underrated albums such as 'A Salty Dog.'
I liked someone else here mentioning Love Affair as they were hugely succesful in the UK in 1968- the only band to exceed them in single sales in 1968 was the Beatles.
That should do for a while. As I like casting light on little known groups, I'll likely come back and list some more later.