I think the 15-30 best centers can be at any point in that range in any given year. It's all subjective anyway. I don't think the difference among the top 30 centers after the first few is all that significant.
The point is, Shattenkirk is in a similar situation on the other side, he is a top 20ish defender, and saying he's just a #2-3 isn't painting the full picture.
I don't mean to bicker because i think a C for D trade between the Bruins and Blues makes a lot of sense considering the strengths and weaknesses of each team, I like the Blues, and am one of the bigger Shattenkirk fans on the Bruins board.
That said, Shattenkirk is a very good offensive PMD and PPQB who needs a solid defensive partner and a little sheltering to be most effective. His value mainly comes from his work on the PP(tied for 4th in PP points, only 66th in ES points for dmen)
Krejci is a #1 C who can be effective against top competition and elevates the play of his wingers and provides excellent 5v5 scoring(13th in the league for centers, 25th for pp points) and is an all situations type player.
IMO a player like Krejci holds more value. I have trouble agreeing that Shattenkirk can be a top 20ish dmen when he's been playing second fiddle to a top 5 dmen and is not a guy i would want against top competition.
Torey Krug is 20th in point for defencmen since he broke into the league in 2013-2014(21st for ES, 18th for PP points)
In that same time Shatterkirk is 17th in points for defencemen(3rd for PP points, 56th for ES points)
Krug's offence, both ES and PP, show you that he is a #1 guy as far as offence goes. But Krug has limitations, simular to Shattenkirk, that would keep him out of the top 30 dmen in the league for me, arguably out of the top 50-60.
I think, despite being a superior ES producer, that Krug is an inferior player to Shattenkirk, but I don't think the gap is big enough for me to put Shattenkirk in the top 20 dmen in the NHL. A top 20 offensive dman, sure, but I don't think a team is going to do much damage with Shattenkirk as thier #1 D.
Would I love him on the top pairing with Chara in his remaining years? yes. Do I want to move the catalyst of the Bruins offence for him? No. Not without a contract and a good plus coming from the Blues. And even then it opens a huge hole at C and is yet another great offensive offensive player shipped out of Boston for a combination of lesser parts which the Bruins have had their fill of over the last few years.