1. The Good Ones
I realize this is a terrible umbrella category, but it covers many films where Martin plays a leading role. In the good ones, his character is engaging, even if not always completely likeable; he is always sympathetic. A good one may be comedic, dramatic, or a combination of the two. At their best, there's a certain originality to the role or the take on the story that stays with the audience.
example -- comedy: The Jerk
The Jerk has reached legendary status for being so off-beat. I almost considered classifying it seperately. Not everyone will necessarily respond to it, but it cannot be denied that it is one of the strongest pieces that he has made
example -- drama: Leap of Faith
I like this movie a lot, and if you haven't seen it, check it out. He plays a con-man healing evangelist, but his plans change when it looks like there's been a real miracle.
other notable films: Roxanne, Parenthood
2. Brilliant Art House
Every now and then in recent years Steve Martin has taken breaks from doing lousy movies that make money and focusing on bizarre smaller films. In these, characters are sometimes much more eccentric but played low-key and never for cheap laughs.
Shopgirl is one of my favorite movies that nobody has seen. Based on his novella of the same name, it is Steve Martin's quirky and delicate look at a woman torn between two very different suitors. For a time Jimmy Fallon was attached to the project, but that fell through and when the film was finally made a few years later, it featured Jason Schwartzman. Clare Danes is radiant.
other notable films: Novocaine
3. The Unnecessary Remake
Steve Martin has made a lucrative career of remaking classic comedy that didn't need remaking. Generally this means finding a classic comedy, filling it with over-the-top "jokes" and slapstick, throwing in a foreign accent, and forgetting what made the original any good in the first place. And I don't blame Cheaper By the Dozen for this trend; it really starts with the big-screen version of Sgt. Bilko.
example: Father of the Bride
While not actually a bad movie in its own right, was there anything about the Spencer Tracy original that demanded a remake? Father of the Bride is probably the best of the remakes, which seem to get progressively worse. ...And with a remake of Topper on the horizon, I'm getting worried.
other notable films: Cheaper By the Dozen, The Pink Panther
4. The Even Less Necessary Sequel
Thanks to the success of Father of the Bride, we have gotten a sequel to every single remake he's ever made (except the aforementioned Sgt. Bilko). The plots are generally more ridiculous, the chemistry of the characters wears thin, and the jokes are staler. They also feature terribly unoriginal titles; just slapping a "2" on it, instead of crafting a title that makes sense. Father of the Bride Part II does not tell you as a title that the movie is about pregnancy (or the rather bizarre angle of mother and daughter pregnant at the same time).
notable films: Cheaper By the Dozen 2, The Pink Panther 2
5. The Scene-Stealing Supporting Actor
Sometimes Steve Martin takes on a crazy role that steals the movie. It's actually something that begins with his television work. When you think of early Saturday Night Live, how can you help but think of "two wild and crazy guys"? And yet he was simply hosting. He was never a regular cast member. He's done similar good work for Tina Fey, including guest spots on 30 Rock. His supporting characters are usually a little more "out there". They can enhance an otherwise mediocre movie (Baby Mama), or complement an already wonderful piece.
example: Little Shop of Horrors
Little Shop is probably the best example of Steve Martin at his kooky supporting best. His turn as the sadistic dentist is one of the most memorable parts of that film. He never overpowers the movie, but he gets a lot of laughs and has a wonderfully bizarre death scene. He really captures what's written for the character, but adds a little spark here and there.
other notable films: It's Complicated, The Muppet Movie (cameo)
6. Shared Billing
There are many movies in which Steve Martin co-stars with other entertainment giants. Some of these are classics, others are abysmal failures. But he has made a number of movies with other notable comedians, even beyond his teaming with Martin Short in the Bride movies or Eugene Levy in Cheaper By the Dozen.
the good examples: Three Amigos, Planes, Trains & Automobiles
the bad examples: Bowfinger, Bringing Down the House
You could probably set aside a separate category for just his cameo work, but I think that they can all fit under the "supporting" category. His work in Prince of Egypt is actually not much of a scene-stealer, but he does get an odd little song in the middle of the movie. Anyway, that's how I roughly see Steve Martin's career. I'll leave it to you to decide where The Man With Two Brains fits in...