For a powerful, universal feel, and to spur people to sing along, limit your melody to the 5-note pentatonic scale.
For example, the C major pentatonic scale includes the notes C, D, E, G, and A. Those same notes comprise the A minor pentatonic scale, with A as the keynote instead of C.
Songwriters often use the pentatonic scale for just part of a song, where they want a primitive and powerful effect. Then they balance it by using the full diatonic (7-note) scale for a more nuanced melody in other parts of the song.
A pentatonic chorus melody is good for a rousing, anthemic singalong. A pentatonic verse is good for an abstract, detached mood.