1. George Harrison - The Simpsons
Back when "celebrity tv cameos" usually meant special episodes where a celebrity would walk out and get huge studio-audience applause and then perform one of their songs or say their catchphrase, The Simpsons inverted the form by bringing in George Harrison then having him utter two sentences while Homer devours brownies in front of him.
Looking back on his time with his band The Be Sharps, Homer reminisces, Then came the greatest thrill of my life...
Harrison shows up for one more line at the end of the episode to rip on Homer's group re-uniting on a rooftop.
2. Hugh Jackman - X-Men: First Class
X-Men: First Class was a near-total-reboot of the X-Men franchise complete with a mostly all-new, younger cast, but the creators still reached out to Hugh Jackman about reprising his role of Wolverine, which he did, for exactly one ad-libbed sentence that earned the film its PG-13 rating:
3. Robert De Niro - Extras
Extras took the very concept of celebrity cameos to another level of craziness, but nothing was more elaborate than this cameo; after establishing for two seasons that Ricky Gervais' Andy Millman character got into acting because he idolized Robert De Niro, Stephen Merchant's hopeless agent character promises to set up a meeting with him, but Andy doesn't believe him and skips out.
So, the show literally booked Robert De Niro to just sit there and play with a novelty pen: