The Hollies needed a decent follow up to If I Needed Someone's failure to ignite the charts. Not a huge hit by their usual standards. I Can't Let Go was a good cover and 66 I think was their best year for 45s.
As for their albums, I'd say that Butterfly and Evolution stand out.
Their best IMO. Underappreciated albums both, self-written also. Concur with you regarding their '66 45s output (including that classic among classics,
"Bus Stop" but also another Clarke/Hicks/Nash gem,
"Stop, Stop, Stop"), but 1964 and 1965 weren't mean years for them in that department either (
"We're Through",
"I'm Alive",
"Look Through Any Window",
"Here I Go Again"...)