I just got into B'z myself and I would definitely recommend getting "B'z The Best XXV 1988-1998" and "B'z The Best XXV 1999-2012." As for songs, Dakara Sono te wo Hanashite is great, Ichibu to Zenbu is awesome, and a personal favorite of mine is Liar! Liar!. Their Official Youtube has samples of a lot of their PVs as well:
http://www.youtube.com/user/Bz?feature=watch