What works best?

The answer to that question is very very complicated.

Basically, the only short answer is: it depends on what you're trying to achieve.

Dischordant sounds are sometimes precisely what you need. Concordant sounds are sometimes precisely what you need.

There is no one pat answer to this question.

What key you are playing in will usually determine what chords and scales are appropriate, USUALLY. Depending on what you what to sound like. But sometimes using dissonance and disharmony in the right way can create tension and relief.

But, generally speaking, yes. The attraction of the blues scales to a beginning improviser is that you can't play any really "bad" notes. That's the case with both the major and minor pentatonics and blues scales over a I-IV-V major key accompaniment. But the major pentatonic or blues scale will produce some "clams" over a minor accompaniment, whereas the minor pentatonic or blues scale still works.

I was also fiddling around with the C Major scale over an E Major chord and wondered whether this would be classed as E Phyrgian ... as E Major's 2nd, 3rd, 6th and 7th notes are flattened to effectively form the C Major scale?

Well, the G natural in the scale of C major will clash quite harshly with the G# in the scale of E. Likewise the C D, and F in C Major will clash with the C#, D# and F# in the scale of E major, and none of the chords in the key of C Major exist in E Major.

If you'd said you were playing a scale of C major over a chord of Em, you'd have been on the right track. Check out the lessons on this site if you don't immediately understand why.

Best

A :-)

Well you could play the blues scale and the pentatonic over pretty much every thing but you probably knew that. also you might want to try doin' model shapes on different keys. my favorite is A Mixolydian. but what ever. you can also do the eddie van halen scale if you know it. also it might be a good idea if you didn't play certain notes in a sclae to make it sound totaly different. also try fusing modes with the blues scale D Dorian blues. i know it sounds weird but its awsome.
GIT SNAKE BIT !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

I was also fiddling around with the C Major scale over an E Major chord and wondered whether this would be classed as E Phyrgian ... as E Major's 2nd, 3rd, 6th and 7th notes are flattened to effectively form the C Major scale?

The reason It sounds so cool when you play a E major chord while playing a C major (or Am) sounding song is because the 6th of a c major scale is A so thats the relative minor. So then if you play a A harmonic minor scale it will change the em chord to an E major chord. The harmonic minor sound is used a lot in music and you can always mix up the minor scales for a more pleasing sound.

The harmonic minor scale is

1 2 b3 4 5 b6 7 1

A harmonic minor

A B C D E F G# A

5 7 2 makes E G# B an E major chord.

Now it could be a lot of other things also, but this will probably give you the results your looking for. You can really mess with the major scale however you want if you play it in the right way.