I find it hard to believe that Messier only won one Smythe award in his career. He should have won it in 1990 and 1994: Here are numbers from those years:
1990: 22 GP, 9 G, 22 A 31 Pts Winner: Bill Ranford
1994: 23 GP, 12 G 18 A 30 Pts Winner: Brian Leetch
In 1990, the Oilers were a very balanced team once they sorted out Winnipeg. Disregarding Ranford, I would say these were the best players in each series:
vs. Winnipeg = the Kurri / Tikkanen / Lamb line was best, so we'll give it to Kurri for most points
vs. Los Angeles = the Simpson / Messier / Anderson line was best, but I'd probably give it to Simpson who was lights out (Messier was great, too)
vs. Chicago = Messier was best here, and single-handedly won game four, which turned it in Edmonton's favor
vs. Boston = probably Simpson again, with a dash of Kurri and Anderson
So, I don't really see Messier as a Conn Smythe winner in 1990. He was superb as usual, but Ranford really deserved it, and for skaters, maybe Craig Simpson deserved it as much as Messier.
The '94 Oiler-Rangers I didn't see as consistently, but from what I remember Brian Leetch really deserved it. (Leetch was by far the Rangers' top scorer in rounds 1, 2, and 4 collectively, though Messier was the best against New Jersey in round three.)