"Obviously I've got a huge knowledge of Stephen's game, having worked so closely with him during my time in charge of Wales," said Ruddock.
"And my honest view is that Stephen has put down an outstanding case to be the first choice Lions No 10 this summer.
"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.
"And he doesn't panic under pressure. In fact, he does the opposite, making accurate decisions when under pressure."
Ruddock added in the Western Mail: "Stephen has developed into a first-class fly-half and is right up there with the very best.
"Of course, it's not my decision, it's Sir Clive's. But, if he plumps for Stephen, he won't be let down."