It looks to me like it's in A minor. The A minor scale works over most of the chords, A harmonic minor over E7. Bb7 is out of key, so your safest option is chord tones (arpeggiate Bb7). You could also arpeggiate any of the other chords.

Also, there's no 'correct' scale. You could play any scale you want, as long as it sounds the way you want it to.
My name is Andy
Quote by MudMartin
Only looking at music as math and theory, is like only looking at the love of your life as flesh and bone.

Swinging to the rhythm of the New World Order,
Counting bodies like sheep to the rhythm of the war drums