I think a lot of people think of Martin as being our #1 because he's been in the side and around for so long that he must be the best, when really he's a pretty darn good workhorse who has managed to stay relatively free of injury, while the rest of the attack chops and changes due to form or injury (and perhaps even ICL bans).
In Mills first 4 Tests he was fairly ordinary taking just 3 wickets. He was dropped IIRC, then recalled in 2006 against the West Indies and between then and now he has taken 41 wickets @ 28.78, including the 2.33 series he's been out of form in (Bang, WI and current Indian).
Certainly in my mind he was our best bowler in the
period between West Indies in 2006 and England in England last year.
O'Brien has taken over now though, no question.