I know @John_Martone was pulling for a Shonen Action Valentine's (& I thought also of compiling a 80's rock/metal AMV list) but decided instead to go with my one true love:
I love you, Beer. You've never let me down. Never change, Beer. Stay by my side... forever. I'll always be there for you, Beer, just like you've always been there for me.
OK, so maybe a couple of love-themed AMVs would be appropriate for Valentines:
Special Bonus --
Not an AMV, just my theme song: