Ross and Rachel first kiss at the coffee house. And they're not even in my top 5 couples, I just think that it's a kiss that most people know, and it was a big deal. I truly think it's iconic.
Definitely Ross/Rachel, purely because nerdy guys everywhere instantly had hope that they could be with the cheerleader they have had a crush on for years but would never have had the courage to tell her how they feel. This kiss cemented, like nearly 10 years of emotions for Ross, especially with his failed marriage, one can only imagine how he would have felt, getting to kiss the girl he loved for so long.