I agree with Newfan about the 70s being their best decade.
But of course there is more to say.
Without discarding the humble beginnings in Oscar Kashama's OK Bar, the band really took off in the early 60s, when Simaro had joined the band and singers like Vicky and Mujos contributed great songs. Later that decade Verckys joined and the overall sound became very exciting, with long guitar-sax sebens that still do the trick of making you want to dance on the spot.
Then, in the 70s, the arrival of the giants: Sam Mangwana, Youlou Mabiala, Ntesa Dalienst, Josky. OK Jazz really became Tout Puissant. Simaro wrote some of his most poignant songs, like Ebala ya Zaire and Radio Trottoir, a song that has also one of the most impressive sebens I've ever heard.
By the end of the decade the classic big band style had taken shape, and it wouldn't change in the 80s, only become more refined. Regardless of what I said about the 70s, some of my favorite songs are from this era, like Kimpa Kisangameni and Testament ya Bowule, both of them timeless masterpieces. And don't forget singers like Madilu and Malage, two of the finest voices that have ever been on one stage together.
All in all, in all of Franco's catalogue there isn't one song that doesn't do it for me. Some are better than others, but there are no bad OK Jazz tracks, from any era. Even the songs from the very first beginning stand out as promises of what was to come. This is a band that almost lasted a lifetime, and will shine on to entertain future generations.