Yes, Monica's proposal to Chandler in Friends still knocks all remaining proposals out of the park, in large part because it expands the character. Monica has always presented herself as someone who expects the boyfriend to make the perfect, utterly romantic, so-amazing-it-could-be-televised proposal. Consequently, her simple but elegant proposal (which is still worthy of being televised) is actually a surprise (as opposed to the designated point in the sitcom where someone/anyone has to get married--okay, it is that too).
Frasier, Niles to Daphne
Niles's proposal to Daphne--the sincere, heartfelt simple proposal despite his big plans--takes a close second.
And naturally, it is hilarious watching Frasier tackle a trumpeter.
Vicar of Dibley, Hugo to Alice
In all honestly, though, my favorite is Hugo's proposal to Alice in Vicar of Dibley. Love the Aliens reference!
Hugo Horton: In the words of Sigourney Weaver in the film Aliens...Last Man Standing, Kyle to Mandy
Alice Tinker: - Get away from her, you bitch!
Speaking of canny dopes, Kyle's proposal to Mandy is lovely, in part because both characters treat the mix-up/confusion that precedes the proposal as the opposite of the end of the world. If this was one of those shows where everyone has soooo much angst, that mix-up would have lasted several seasons.
On the other hand, a little disappointing....
Golden Girls, Lucas to Dorothy
It's fun, but it is still disappointing that the show threw in Leslie Nielsen at the last second. I mean, okay, yes, Leslie Nielsen! But still--it would have been nice if he had showed up half-way through the last season. Granted, the hate-love relationship trope can only go so far; 1/2 a season would have been enough.
On the other other hand...
Golden Girls, Can't Remember His Name
There is a proposal in Golden Girls where a young man proposes to his boyfriend after listening to Blanche tell the story of her courtship. I like this scene because it reminds me of a scene from Will & Grace, in which Grace's boyfriend (played by Woody Harrelson) oversees a proposal between two men in a bar (hey, it could have been the same bar! okay, not really) and realizes that relationships do not have to be as unbelievably difficult as his relationship with Grace. I discuss that more here.