If you type 'top 10 Valentine's day movies' or similar in a search engine, you will find several different lists to give you ideas. Everyone has their own favourites of course, and actually you can subdivide romantic movies into different types - comic or tragic, for instance.
If you're looking at light love comedies or romcoms, popular choices include When Harry Met Sally, Love Actually and, very aptly named, Valentine's Day, which is about love stories seen from lots of different angles. Or if you prefer a weepie there is Truly Madly Deeply or, if you want a famously sad romantic classic, Love Story.
For something different, with real feeling, you could try the Time Traveller's Wife - or the most enduring love story of them all, Romeo and Juliet - if you haven't seen Baz Lurhman's version it's as watchable as any modern film while still keeping the glories of Shakespeare's language. Or for a feelgood film with a bit of magic and fantasy, there is Spellbound or - not usually seen as a love movie but genuinely romantic - Groundhog Day.
You can get some more ideas here.