1. I’ve included the movies that have come out in the past year (X-Men: First Class, Thor and Green Lantern).
2. A few people thought that it would be fairer to look at only the current wave of superhero movies (starting in 2000 with X-Men).  I’ve added a section comparing both companies’ performance post-2000.

Summary

• Including the older movies, the average Rotten Tomato score was 50.2% for DC and 58.1% for Marvel.  If we look only at the modern movies, the gap narrows somewhat.  Since 2000, DC has averaged 54.8% and Marvel has averaged 59.9%.

• Marvel has been having more critical success with more series.  Since 2000, DC’s non-Batman movies have averaged 47.1%.  Since 2000, Marvel’s non-Spiderman movies have averaged 55.8% and its non-X-Men movies have averaged 56.4%.

DC Movies

A-Grade (80+ on Rotten Tomatoes)
The Dark Knight-94%
Superman-94% *
Batman: Mask of the Phantasm-87% *
Batman Begins-85%
Superman II-83% *

B-Grade (60-79 on Rotten Tomatoes)
Batman Returns-77%*
Superman Returns-76%
V for Vendetta-73%
Batman-71%
Watchmen-64%

C-Grade (40-59 on Rotten Tomatoes)
The Losers-47%
Constantine-46%
Batman Forever-44%*

D-Grade (20-39 on Rotten Tomatoes)
Green Lantern – 25% (after three days of reviews–this figure may change slightly as more reviews come in)
Superman III-23% *

F-Grade (19 or less on Rotten Tomatoes)
Steel-13% *
Jonah Hex-12%
Batman and Robin-12% *
Superman IV: The Quest for Peace-11% *
Catwoman-10%
Supergirl-8% *

*–These movies came before 2000.

Marvel Movies

A-Grade (80+ on Rotten Tomatoes)
Spiderman 2-94%
Iron Man-93%
Spiderman-90%
X2: X-Men United-88%
X-Men: First Class-87%
X-Men-81%

B-Grade (60-79 on Rotten Tomatoes)
Thor-77%
Iron Man 2-73%
Incredible Hulk-66%
Spiderman 3-63%
Hulk-62% (How!?)

C-Grade (40-59 on Rotten Tomatoes)
Blade II-58% *
X-Men: Last Stand-57%
Blade-55% *
Daredevil-44%

D-Grade (20-39 on Rotten Tomatoes)
X-Men Origins: Wolverine-36%
Fantastic Four-36%
The Punisher-30%
FF: Rise of the Silver Surfer-26%
Ghost Rider-26%
Blade Trinity-26% *

F-Grade (19 or less on Rotten Tomatoes)
Elektra-10%

*–These movies came before 2000.

Here are some other ways to look at the data.

ALL OF THE ABOVE MOVIES (REGARDLESS OF YEAR)

Including the older movies, the average Rotten Tomato score was 50.2 for DC and 58.1 for Marvel.

ONLY MODERN MOVIES (SINCE 2000)

Excluding the older movies, the average Rotten Tomato score was 54.8% for DC and 59.9% for Marvel.  That’s closer, although Marvel movies are still a bit better on average.

Some other trends that stuck out.

• In the modern era, Marvel is having more critical success with more series. In the modern era, DC averaged 54.8% with all movies included, but 47.1% in non-Batman movies.  Marvel averaged 59.9% with all movies included and 55.8 in non-Spiderman movies and 56.4 in movies without X-Men.  In the modern era, DC has had 4 non-Batman movies rated A-grade or B-grade.  Over the same period, Marvel has had 8 non-Spiderman and 8 non-X-Men A-grade or B-grade movies.  
• Hollywood is still as inept with female leads as it was 25 years ago. Elektra (10%) and Catwoman (10%) turned out as bad as 1984′s Supergirl (8%).  That probably can’t just be attributed to budget: Catwoman had a production budget of $100 million in 2004 and Elektra had $43 million in 2005.  According to IMDB, Supergirl’s budget was $35 million, which (if adjusted for inflation) works out to about $80 million in 2005.  Other films did much better with comparable budgets, such as X-Men ($75 million in 2000).  That said, most of the above movies had budgets between $100-200 million.

Questions? Concerns?  Other comments?  Please let me know and I might incorporate them when I update the survey with more recent releases.

For the sake of convenience and clean numbers, I took the top 20 grossing movies from each publisher and then gathered their Rotten Tomato rankings, which are averages of hundreds or thousands of reviews.  (A RT ranking isn’t a perfect measure of quality, but it’s probably pretty accurate).

DC Movies

A-Grade (80+ on Rotten Tomatoes)
The Dark Knight-94%
Superman-94%
Batman: Mask of the Phantasm-87%
Batman Begins-85%
Superman II-83%

B-Grade (60-79 on Rotten Tomatoes)
Batman Returns-77%
Superman Returns-76%
V for Vendetta-73%
Batman-71%
Watchmen-64%

C-Grade (40-59 on Rotten Tomatoes)
The Losers-47%
Constantine-46%
Batman Forever-44%

D-Grade (20-39 on Rotten Tomatoes)
Superman III-23%

F-Grade (19 or less on Rotten Tomatoes)
Steel-13%
Jonah Hex-12%
Batman and Robin-12%
Superman IV: The Quest for Peace-11%
Catwoman-10%
Supergirl-8%

Marvel Movies

A-Grade (80+ on Rotten Tomatoes)
Spiderman 2-94%
Iron Man-93%
Spiderman-90%
X2: X-Men United-88% (How!?)
X-Men-81%

B-Grade (60-79 on Rotten Tomatoes)
Iron Man 2-73%
Incredible Hulk-66%
Spiderman 3-63%
Hulk-62% (WTF? I would put this at 20, tops)

C-Grade (40-59 on Rotten Tomatoes)
Blade II-58%
X-Men: Last Stand-57%
Blade-55%
Daredevil-44%

D-Grade (20-39 on Rotten Tomatoes)
X-Men Origins: Wolverine-36%
Fantastic Four-36%
The Punisher-30%
FF: Rise of the Silver Surfer-26%
Ghost Rider-26%
Blade Trinity-26%

F-Grade (19 or less on Rotten Tomatoes)
Elektra-10%

Here are some other ways to look at the data.

The average Rotten Tomato ranking was 51.5% for DC and 55.7% for Marvel. So the movies on the whole tend to be pretty comparable, although most of the truly horrendous movies were DC.

Some other trends that stuck out.

• Except for possibly Ironman, all of the excellent movies came from the largest franchises (Batman, Spiderman, X-Men and 1980s Superman).  My theory here is that studios are far more likely to spend on quality actors, screenwriters and directors if they think the franchise has a large-enough fanbase to support the investment.
• Movies based around teams or multiple characters tended to do slightly worse than ones with a more singular focus. Team movies comprised just 16% of the movies that were A-grade or B-grade (3 out of 19).  But they comprised 20% of the total sample.
• The three movies centered on a solo female protagonist were unmitigated disasters. Supergirl, Catwoman and Elektra weren’t just in the F-grade, but the three worst of all forty surveyed.  Some possible explanations: the studios made a half-assed effort because the franchises were minor and/or because they didn’t think superheroines would sell well.  Audience discrimination might have played a role, but women reviewers on RT seemed to hate these movies as much as the men did.
• The five movies with black leads (the Blade trilogy, Steel and Catwoman) scored an average of 32%, worse than average (53%) but much better than the movies with female leads (9%).  I’m a bit surprised black protagonists fared worse than average because Wesley Snipes is actually pretty decent, Halle Berry has won an Oscar, and the Blade movies made enough money (half a billion dollars) to justify substantial studio support.  PS: Whoever cast Shaq as Steel is going to the same circle of hell as the guy that put nipples on the Batsuit.

• Disney is paying roughly $50 per share, which is a 29% premium over Friday’s closing.  If you own Marvel stock, you will come out ahead quite nicely on this.  It was trading around $25 earlier this year.
• I am cautiously optimistic that Disney knows how to buy a successful firm without ruining what made it successful.  For example, Pixar’s movies didn’t drop in quality after the Disney buyout.  (Nor have they released a lot of straight-to-DVD sequels to successful movies).
• I doubt this will have a noticeable impact on Marvel’s products.  Even the movies.
• I think Disney is the biggest loser here.  It’s betting 4 billion dollars that it can leverage Marvel’s characters better than Marvel did.  I’m skeptical.