I don't know about favorite, but the ones that I watch OVER and OVER are:

You've Got Mail

Runaway Bride

One Fine Day

Sex and the City (does that count?)

And recently, I LOVED The Ugly Truth!

How to lose a guy in 10 days, and Sweet Home Alabama.  I could watch those 2 over and over again.  Two Weeks Notice too.

Oh yeah!  I love those too TG!  :)

Roman Holiday

Heart and Souls

Love Actually

Hitch

50 first Dates

When Harry Met Sally

There are a ton... But one of my all time favorites:

How to Lose a Guy in 10 Days

Also, Family Man with Nicholas Cage.. Ugh.. :)  LOVE!!

The Wedding Singer =)

Love Actually & The Holiday are some of my faves.

I don't know about my "all time" fave.

I love LOVE ACTUALLY too!  I guess I have a lot of favs.  :)

Love Actually is at the top of my list

Sabrina, Say Anything, When Harry Met Sally, Return to Me, Keeping the Faith, so many more

He's Just Not that Into You...Hitch