Given that I'm a stone fantasy fan, it's probably inevitable that I'm going to select more crossovers with fantastic, as opposed to realistic, themes. However, personal taste isn't the only reason I tend to rate the fantasy-crossovers higher than the reality-crossovers-- many examples of which are found here-- though it would take another essay to address those reasons.
However, here's one of the great reality-themed crossovers:
COUSIN MAUDE'S VISIT
Almost as soon as producer Norman Lear vaulted into major success when ALL IN THE FAMILY became a critical and commercial success, Lear capitalized on that success by creating not one but two "back door pilots" in FAMILY's second season. In December 1971 Edith Bunker's cousin Maude came to care for the Bunker family at a time when Archie, Mike and Gloria were all sick and running Edith ragged. In March 1972 Archie and Edith visited Maude's house, giving the producers the chance to introduce FAMILY's audience to Maude's husband and daughter, and to show the actress Bea Arthur to much better effect: as the domineering diva of her household.
To be sure, neither episode is the best either show could offer, nor are the crossover elements sustained to the best effect. "Maude" is the better written of the two, precisely because Archie and Edith are barely in the story, allowing the script to introduce the ethos of ultra-liberal Maude and her unconscious lapses into chauvinism, seen in her attitude toward her daughter's prospective Jewish husband.
In contrast, "Cousin Maude's Visit" is a very thin story, but it does have some classic confrontations between Archie and Maude, as when Archie gets Maude's goat with his classic line, "Franklin Delano Roosevelt ruint this country." To be sure, Maude doesn't get a great comeback, reduced to the response, "You're fat." Still, the episode does put across a broad, quasi-mythic conception of the inherent conflict between a White Working-Class Conservative versus a White Arch-Liberal/Feminist.
I don't think the show MAUDE ever reached the heights of FAMILY at its best. Yet MAUDE did spawn a show that reached those heights on occasion, though it's arguable that the "Florida Evans" of MAUDE is not quite covalent with the materfamilias on GOOD TIMES.