He sure was, in the regular season. One of the great, certainly. Guaranteed Hall of Famer. Unparalleled Vezina domination. Vezina doesn't mean great. Dominance in the regular season doesn't mean great. Look at it over the same period the two men were active, 1993-2002, 9 seasons (excluding the 1992 season, where Hasek played one game, that would be unfair to add in there). This is during Hasek's dominance of the Vezina.
Patrick Roy: GAA: 2.19 S%: .921
Dominik Hasek: GAA: 2.00 S%: .928
It should be noted that Hasek played in approximately half the games Roy did, so Roy played more of his games later in the playoffs, when goal scoring is statistically higher as players get sloppier from the grind - and as you play against better teams. I think these are comparable numbers, when you consider that. And when you consider Roy went all the way three times in that period and Hasek went once (with a super duper team)...
And yeah, Moose was never the best in stats, he just played 24 solid seasons in the highest scoring period in NHL history. Helps for your points margin.
And finally, it comes down to your definition of great. If your definition is numbers? Hasek is probably going to win every time. I don't define greatness just by numbers. It's also accomplishments. It's also pizazz. It's hardware, and it's rings. Roy hangs close to Hasek with the numbers, equals him with pizazz, and blasts by him with rings and hardware (if you count the Conn Smythe as more than Vezinas...which I do). Hasek is close. Easily in the top 3. But he's not Roy.