Links: TVTropes, Wikipedia.
The series, while retaining the "slasher film" concept throughout, can be roughly separated into two distinct groups: the early films are "campfire story" slashers (where Jason is implied to be a deformed loner psycho man-child), while the later films are "supernatural serial killer" slashers (where Jason is implied to be some sort of ghoul or super-zombie). Very few of the films have any merit as actual horror movies; only the first film makes a noteworthy attempt. But most of the films have entertainment value for the comedically gruesome deaths, starring Jason as the Villain Protagonist, and some of the films have merit as outright comedies. The films are perhaps best enjoyed by watching while performing one or more of the activities that get the "protagonists" killed (i.e. partaking of alcohol, pot, and premarital sex).
Friday the 13th (1980, 6.3)
The film is an early member of the slasher film genre, predating Nightmare on Elm Street by 4 years, and as such it's pretty dated. But it's fairly watchable, does a decent job of building tension, and has a small but nice twist as it builds to the climax. Score: 3 stars out of 4.
Friday the 13th Part 2 (1981, 5.6)
Yawn. This film is noticeably more dull than the previous one and has very few redeeming qualities. That said, it's not unwatchably bad. Score: 2 stars out of 4.
Friday the 13th Part III (1982, 5.1)
Ugh. For a very long time, this would be the worst film in the series. Notable for being the first (and last) film in the series to be released in 3D, with exactly as much "theme park" camerawork as you might imagine from a film trying to cash in on the 3D revival of the late 70s and early 80s. Trivia: first movie where Jason acquires the iconic hockey mask. Score: 1 star out of 4.
Friday the 13th: The Final Chapter (1984, 5.3)
A surprising buck of the trend, this film suddenly leaps into "solidly watchable" territory. This is the first film that seems to revel in self-awareness of its audience, and as such it has far more humor than the previous entries. TVTropes calls it "a So Bad Its Good teen comedy that morphs into a So Cool Its Awesome horror movie".Trivia: this introduces Tommy Jarvis, the most likable hero in the series, and starts his protagonist arc. Score: 3 stars out of 4.
Friday the 13th: A New Beginning (1985, 4.1)
I have mixed feelings about this. Plot-wise, this movie is dumb, dumb, dumb, and completely pointless with regard to the rest of the series. However, this movie further refines the self-aware humor of the previous film and actually spends a lot of time trying to make the audience laugh. Score: 2 stars out of 4.
Friday the 13th Part VI: Jason Lives (1986, 5.3)
Best film of the series, full stop. The self-aware humor of the previous two entries is refined to perfection, with most of the cast playing genre-savvy deadpan snarkers. In addition to concluding the Tommy Jarvis arc, this film also marks the point in the series where Jason becomes explicitly supernatural. Trivia: includes the musical talents of Alice Cooper, who created songs just for this movie. Score: 4 stars out of 4.
Friday the 13th Part VII: The New Blood (1988, 4.7)
A decline from the heights of the 6th installment, but still halfway decent. Introduces Tina the Psychic Girl, who surprisingly did not have a protagonist arc of her own but instead was confined to one movie. I've heard contradicting claims that this movie was either (a) salvaged from an early "Freddy vs. Jason" script, or (b) written as a stopgap or lead-in for such a movie. Either way, Tina would have been a natural protagonist for such a film as she's a more natural genre fit for Nightmare on Elm Street than she is in this series. Score: 2 stars out of 4.
Friday the 13th Part VIII: Jason Takes Manhattan (1989, 3.9)
Every bit as horrible as it sounds. Score: negative 4 stars, a repair bill for breaking my scale, and my everlasting hate.
Jason Goes to Hell: The Final Friday (1993, 4.1)
This film marks the sale of the series from Paramount to New Line Cinema, the latter being the home of Nightmare on Elm Street. This sale was presumably in anticipation of a "Freddy vs. Jason" film, but the rights to the "Friday the 13th" name slipped between the couch cushions in the transition. The film itself is quite bad, but not unwatchable. The first 10 minutes or so capture the precise flavor of "awesome" sought and treasured by 14 year old boys, but the rest of the film is a complete miss with its out of left field plot elements and the addition of bizarrely creepy pseudo-sexual gore. (Worst, they nonetheless fail to reach David Cronenberg heights, so what's the point?) Score: 1 star out of 4.
Jason X (2002, 4.4)
Also known as "Jason... IN SPACE!" A sudden return to self-aware humor, to the extent that it works as a solid sci-fi comedy. The film includes some delicious mockery of the earlier films, a wonderfully snarky sense of humor, and some So Bad It's Good action sequences, some of which can be found on YouTube. Unfortunately the latter part of the movie is bogged down by pacing problems, but by my reckoning it's nonetheless the second-best film in the series. Score: 3 stars out of 4.