I'd say Pietrangelo deserves it. He was here for over a decade, was our #1 defenseman for nearly his entire time here, and was arguably the most important player overall for our Cup run. And his game 7 is among the best single game performances in Blues history, and it's obviously the most important. I understand why people don't feel right putting 27 in the rafters right now, but I can almost guarantee that after Pietrangelo retires, most of you will have warmed up to it.
However, it must be said that Pietrangelo is my favorite all time Blue, and has been for nearly a decade now. So there is bias. Regardless, the numbers and the results don't lie. Among Blues defenseman, he's 2nd in games played, 2nd in goals, 1st in assists, and 2nd in points. If you don't think those things warrant 27 being retired, then a Stanley Cup certainly puts him over the edge.
If Tarasenko decides to stay, and if his play going forward proves that the last few years were simply "a bump in the road," then he's got a real good shot (pun intended). He'd almost certainly slot right into 3rd all time in points and 3rd in goals (currently 159 points and 71 goals back). And him and Perron are the only current players with a real shot at sniffing 2nd or 3rd in games played.
O'Reilly could be in the discussion depending on how the rest of his career goes, but a lot of things would have to go right.
TL;DR: Pietrangelo should be, Tarasenko, O'Reilly, and Perron are in the picture, not so sure anyone else is.