harmonic minor - for metal, neo-clasical stuff, some ritchie blackmore's songs and probably other things too
the mixolidyan mode - if you like rocknroll or blues you should learn this one, almost every rocknroll riff from the 50-60 was based on this scale. this mode is also goes well with the dominant 7 chord, and combining it with the appropriate ionian (major) scale is one of the basic things in harmony