The playoffs are where goalies make their reputations, and the spotlight is never brighter than in the Stanley Cup Finals. Broadcasters and other media types ramble about the importance of goaltending, and depending on the final result assign either all the credit or all the blame to the goaltender.
If goaltending really is so important, however, then the team with the better goalie should have won a large majority of the Stanley Cup Finals. I went back and looked at all the Cup Finals since 1968 (the expansion era). For some of the seasons I have game-by-game save percentage statistics (courtesy hockeygoalies.org), which allows for an even more detailed analysis.
My goal was to identify the series where a goalie was the difference in the series. That is, either an outstanding performance by a goalie on the team that was worse during the season but still won the Cup, or a terrible performance by a goalie on a good team that ended up losing the series. It didn't really matter so much how well the goalies played, just whether or not the inferior team ended up winning.
For example, in 1996 Patrick Roy had an outstanding Cup Final, helping Colorado sweep Florida and posting a .974 save percentage while outplaying John Vanbiesbrouck in every game. But Colorado finished 12 points ahead of Florida in the standings that season, outscoring them by 72 goals with almost the same defensive record. Clearly, Colorado was a much better team, and despite Roy's performance he cannot be credited with winning the series.
I elected not to use points alone to determine which team was better, because luck can often play a role in determining a team's final position in the standings. I also considered goals for and goals against, to break the tie if teams finished with a similar amount of points.
I found only 7 instances over 39 seasons where the team I judged to be worse was triumphant in the Stanley Cup Finals. Over 80% of the time, the better team won. This is strong evidence that, even in the playoffs, the team contribution is much more valuable than the goaltender's contribution to the final result. Obviously, the goalie's contribution is also included in the team's position in the standings, but most of the time there was a clear difference between the quality of the participants.
Here are the times when the better team lost the Stanley Cup Final:
2001: New Jersey loses to Colorado
1995: Detroit loses to New Jersey
1990: Boston loses to Edmonton
1985: Philadelphia loses to Edmonton
1983: Edmonton loses to N.Y. Islanders
1980: Philadelphia loses to N.Y. Islanders
1971: Chicago loses to Montreal
I looked at the three that I had detailed information on. In 2001, Patrick Roy was outstanding (.938) while Martin Brodeur was poor (.870), and the Devils lost despite outplaying and outshooting the Avalanche. In 1995, Brodeur played well against a strong Detroit team (.913), but the decisive factor was Mike Vernon's terrible play at the other end (.846). In 1990, Edmonton's Bill Ranford stoned Boston, posting a .949 save percentage despite being outshot by nearly 5 shots a game, outplaying the tandem of Andy Moog and Reggie Lemelin (.846 combined).
For the other 4, I suspect that two of them include outstanding goaltending performances, based on the goalies receiving the Conn Smythe Trophy (Ken Dryden in 1971, Billy Smith in 1983). As I posted on before, however, Smith's performance was greatly assisted by the strength of his team, but I'll count that one just to be on the safe side. The other two were likely outliers. The Oilers were clearly a dominant team in the middle of a run of 4 Cups in 5 years, and had a bit of an off-season in 1985 when they had "only" 109 points, a season sandwiched by two years with 119. The Islanders were just beginning their Cup run, and were likely a much better team during the 1980 playoffs than they performed during the season. This is suggested by their 19 point improvement in the standings in 1980-81. It could be that there were outstanding goalie performances in either of those years, but I doubt it.
I went through the years from 1986 to 2007, and estimated that nearly half of the time the better goalie ended up losing. The better team won 17 out of the 20 (85%).
This analysis suggests that in the 39 seasons since expansion, the only goalies to steal a Stanley Cup Finals series were Patrick Roy, Bill Ranford, Ken Dryden, and possibly Billy Smith. The vast majority of the time the better team won, and on several occasions, the winning goaltender was actually outplayed by the losing goaltender. This indicates that even in the playoffs, where goaltending is supposedly so important and where players are labeled as "clutch" or "chokers", almost all of the time it is the best team that ends up on top.