I just finished going through my doors and installing new speakers, among other things.
I wanted improved sound for sure but nothing too wild in terms of fabricating anything or cutting or drilling into the doors.
In the rear doors I used Retrosound R452-N, fit the speaker baskets perfectly; the only difference is there is no screw hole in back. I tested various ways to attach the speakers to the baskets, in the end I used FlexGlue to glue them in, working great. The speakers fit the stock grilles perfectly. Sound vastly improved.
In the front I used Polk Audio db-651s speakers. I made a pair of 1/2" plywood spacer rings to fit the speaker indent in the door and removed the stock grilles; the holes in the Polk grilles line up perfectly with the slots the stock grilles attached to. Most people around here seem to think the Polk grilles are ugly, but I like them.
Topped it off with a Sony XAV-AX5000 and now I am rocking down the highway.
Decide what lengths you are willing to go to and that will determine your options. Really you can make just about anything work limited only by your imagination and your ability/desire to alter and/or fabricate to make things fit (and budget). I read every frapping speaker/stereo thread on this forum and this is what I came up with that I thought would be best for me, and I couldn't be happier.
It's fun, good luck with the process!