Rielly is definitely moving into the top 5 offensive defenseman in the game. His offensive instincts, passing, skating, positioning, etc, are lethal for him to be the 4th man in and create offense, and he does it at an elite rate. This is nothing new, there have just been small improvements, more confidence, and more opportunity for Rielly over the last few seasons. This season he has definitely moved into the top five, probably trailing Burns, Karlsson for sure, and on the level of guys like Letang, Hedman, Carlson, probably ahead of the Krug, Klingberg, Josi, Byfuglien, Barrie kind of range.
He also might be rivaling the top 5-10 transition defenders in the league. He moves the puck up the ice very well, both passing and skating, and really helps the team get out of the d-zone, and into the 0-zone.
The reason he is not a top 5 defenseman overall is his weak defensive play. Rielly is still not a very good defender when in his own zone. Not awful for sure, but not at the level of other top ten defenders.
Right now, overall, I'd say Rielly probably slots somewhere around #15 among all defenders.