What is the most common mood used in English: indicative mood, imperative mood, or subjunctive mood?
In my opinion it is indicative mood. See explanation for details.
I haven't made any research in this matter, but logically thinking it is indicative mood.
The different moods mean different purposes of sentence. The indicative sentence informs about some (not necessary true or real) facts, the imperative mood means giving orders or commands, subjunctive sentence brings assumptions and hypothesis.
The main purpose of language is to inform, so the indicative mood (giving information) is in my opinion the most frequently used one.