Although I don't, in general, see the point in discussing well-known music, I will make an exception with Gladys Knight - for a possibly contentious reason.
Gladys Knight And The Pips were best known for their 1973 hit Midnight Train To Georgia, although their career stretched for decades either side, and they hit with Heard It Through The Grapevine before Marvin Gaye.
Knight and the Pips had minor hits in the early 60s, then subsequently signed with Motown. Although they hit with Grapevine, they were always treated as second stringers, and so left for Buddah Records in 1973.
I have in front of me two of their compilations: one from their Motown catalogue, and one from Buddah. I was listening to their Buddah compilation and thought "that woman can really sing".
So I turned to Motown. But I just didn't get the same feeling. Thus my contention: their move to Buddah somehow brought about a significant boost in the quality of Knight's performance - and even the Pips come off well.
Midnight Train is a pleasant listen. Their followup, I've Got To Use My Imagination, doesn't move me as much - although Knight's gutsy performance must be well appreciated. Then followed You're The Best Thing That Ever Happened To Me: schmaltzy, but again a bravura performance. Two singles later, You're The Best Thing That Ever Happened To Me again hit a peak.
A variety of styles, and Knight always impresses. She poured in a lot more substance, more emotion than she had in the past. Even the Pips, who only ever came along for the ride, do pretty well on all these pieces.
The three albums these songs came from were all produced by different people. The material was drawn from a more diverse stable of songwriters than in the past, including country writer Jim Weatherly. Although that gave Knight the opportunity to spread her wings, it seems to remain that she put a good deal more oomph into her work upon first leaving Motown.
Why? Many reasons for outcomes dwell behind the scenes; this answer may remain inscrutible. The usual reasons - production, songwriting - seem absent here. It may be that management or executive production from Buddah was the driving force.
I think it's worth drawing attention to this dichotomy. For the best of Knight, you only need turn to the Buddah recordings.
Footnote: apparently the title (but not the story) for Midnight Train To Georgia derived from Lee Majors commenting to Weatherly about the then-unknown Farrah Fawcett catching a midnight plane to Houston.