Quite hard to pick their best era, unless you're strictly 60s or 70s only. I still love Shout, was that 50s, can't remember now? Respectable from that era is great too. I suppose my preference is for their Motown, This Old Heart Of Mine has great sentimental meaning for me, the first present Lesley bought me. But they did have the Motown sound, weren't quite as distinctive as they were on other labels, but the quality of the material is so high.
They made an appearance on Ready Steady Go prior to signing for Motown, sang their Atlantic single, The Last Girl, lovely piece of deep soul, worth listening to imo.
I like their 70s material, play Work To Do often when I get a set. Also like their disco tunes, prefer That Lady to earlier version. Summer Breeze, Harvest For The World and Between The Sheets were great. And the Body Kiss album is pretty good.
All in all they were a premier soul group with their 60s material just shading it for me, probably my favourites are This Old Heart Of Mine and The Last Girl.