Star Trek: The Original Series. Those original Klingons looked essentially human, though with some unfortunate brownface makeup.
The Klingons were briefly featured in Star Trek: The Motion Picture, where they were given a decidedly more alien look, with pronounced forehead ridges and sharpened teeth. They were slightly refined for Star Trek III: The Search For Spock, where the Klingon Kruge served as the primary villain. Kruge and his followers had a major influence on the Klingons featured in TNG. Lieutenant Worf, in particular, would become the face of the Klingon race not only on TNG, but in pop culture in general. That version of the Klingons endured until Discovery season 1, where they were radically redesigned as hairless, oddly colored monsters. That version was immediately reviled, and they haven't appeared in Discovery in any capacity since season 2.
Why Discovery Must Bring Back TNG Style Klingons
The Klingons are a cornerstone alien species in Star Trek. Their absence in the live action Star Trek shows since Discovery season 2 has been conspicuous, perhaps most of all in Discovery's new 32nd century setting. Their absence in season 3 was somewhat understandable, as the crew of the Discovery spent much of that season simply getting their bearings in their new far-flung future. Discovery season 4 dealt with galactic politics revolving around the Dark Matter Anomaly, and the fact the Klingons were completely uninvolved felt wrong.
Star Trek: Prodigy. Their long absence from Discovery seems a tacit ission that the Discovery season 1 version of the Klingons didn't work, so there should be no practical or creative barrier to the return of the TNG style Klingons.
How TNG Klingons Would Link Discovery & Picard
The Klingons didn't appear in the first two seasons of Star Trek: Picard, though that's set to change in a major way. Picard season 3 will see the return of the beloved crew of the Enterprise-D, including Worf, who looks almost exactly like he did in his TNG days, though now with a luxurious mane of white hair. Despite their absence in the series so far, it seemed a no-brainer that Picard would return to the classic Klingon look.
Star Trek: Discovery returning to TNG style Klingons would link it with Picard - and the vast majority of Star Trek fiction - visually, creating a more cohesive feel for the characters. The Discovery season 1 Klingons could be dismissed as a regional anomaly - much like the differing looks for the Romulans were finally explained in Picard sesaon 1. They could also simply be retconned away and forgotten. Star Trek: Discovery would do well to leave that failed experiment behind and embrace the version of the Klingons that have prospered for so long in Star Trek.