Well, you have several options. As a matter of fact, all the DVDs could meet your requirements, there.
If you want the music video for Somebody to Love, get Greatest Video Hits I. If you want the music video for I Want to Breat Free, get Greatest Video Hits 2. If you want to see Somebody to Love played live, get We Will Rock You. If you want to see I Want to Break Free live, get Live at Wembley.
All four of these are excellent. If you can only get one, you should probably start with Greatest Video Hits I, unless you want to see a concert, in which case start with Live at Wembley.