While Johnny Wilkinson battles back from injury, the fly-half spot remains up for grabs, and Ruddock believes the Welshman deserves his chance after his team's memorable Six Nations triumph.
"Obviously I've got a huge knowledge of Stephen's game, having worked so closely with him during my time in charge of Wales," Ruddock told the Western Mail.
"And my honest view is that Stephen has put down an outstanding case to be the first choice Lions No 10 this summer.
"It's not a question of bias, because I try to give a proper, reasoned judgment."
"I think Stephen has proved, during his time in France and also with us in the Six Nations, that he is just about the complete No 10 these days.
"Every facet of his game has come on. He knows how to control a top international rugby match.
"His kicking from hand is superb, his goal-kicking is excellent, he gets his back-line going, as we saw with Wales in the Six Nations.
"He makes great breaks on his own, as we saw with that 60-yard run which helped us get back into the game against France in Paris.
"And he doesn't panic under pressure. In fact, he does the opposite, making accurate decisions when under pressure.
"Stephen has developed into a first class fly-half and is right up there with the very best."