I have to go with Swingers and 40-year old virgin, by a mile.
Unlike the rest of the movies you guys are mentioning, the two have a lot of realism to them. Swingers is almost 100% real life and 40-year old virgin was almost all ad-libbed. "Pu$$y on a pedastool", lol. Almost every great line in there is from real life & experience.
The other movies, ehh, all fantasy, but those two you can actually relate to and learn from.
I remember watching Swingers once with my GF at the time. We're at the part about "Why won't she call?" where Mikey keeps wanting his ex to call but she won't until he actually forgets about her. My GF says out loud "So true".
Funny thing is that she breaks up with me few months later, real out of nowhere, hit me hard. I was Mikey for like at least a month. When I finally totally forget about her, over a year later, she contacts me. Now she chases me, but my interest is long gone. Just like in the movie.