Genesis with Peter Gabriel were pretty cool, but it wasn't until they brought Phil Collins out front to sing that they became huge on the charts.
Michael McDonald was a good example.
AC/DC were already on their way with Bon Scott with a loyal following, but it wasn't until 'Back In Black' with Brian Johnson singing that they started breaking America and hit new heights.
Deep Purple when they switched to Ian Gillan on lead vocals they recorded their most memorable songs (Black Night, Smoke on the Water etc).
John Frusciante - once they got him on board the Red Hot Chili Peppers on guitar, he single-handedly transformed their sound and gave them the classic rock feel that took them to that next level of success.
The Human League were originally just an alternative electro-synth outfit and didn't have any chart success until Phil Oakley added those 2 female co-vocalists around 1980 and the hits flew out from that point on (Don't You Want Me, Keep Feeling Fascination, Human etc).
Same goes for Ultravox. A similar sound to the Human League early on - very electronic and experimental, very punk, but it wasn't until Midge Ure took over the reins that they became huge (Vienna etc).
Fleetwood Mac when they brought in Buckingham and Nicks in of course - that's an obvious one. No offence to Peter Green and Bob Welch - both great artists in their own right but wow, bringing in that particular duo just took the band to a whole new stratosphere.
Split Enz once Neil Finn joined his brother Tim - suddenly the hits were raining out everywhere. Til then they were progressive and arty and glammy but Neil brought out the pop hit element and those Finn brothers churned out the hits like a Motown factory!