I guess I'm wondering what composers people think have the most developed and unique "voice" or compositional style.

My picks are:

Philip Glass (the obvious choice)

Morton Feldman

George Gershwin

Maybe Steve Reich, but not including any of his early music

Top 10 (( for me and my limited classical knowledge anyway! ))

Bartok
Stravinsky
Berg
Debussy
Liszt
Schoenberg
Beethoven
Wagner
Mozart
Bach

bach and mozart? i can't tell them apart for the life of me. i should have put debussy and schoenberg though

All the greats, be they musician/composer and no matter the genre, have their own recognizable style/sound.

come on, humor me. which ones sound the least like anyone else

Phillip Glass :P

i'd say, maybe... zoltan kodaly. good composer who wrote in the early to mid 20th century. thing about him i've noticed is that he regularly incorporated elements of jazz in his works to a lesser degree than gershwin, but nonetheless reflecting the changes in popular music at the time. (opus#4 and opus#8 for cello)

or... toru takemitsu who i've just begun listening to recently, and who had a freeform almost controversial writing philosophy (ala john cage) and frequently uses natural sounds in his recordings. very distinctive. (rain-tree sketch for piano)

Erik Satie ( i would think)

That's pretty bad, blah.

but listening to more of each would certainly help that.... you have to train your ears to listen to classical music (imo), and it wouldn't hurt to have a connoisseur nearby to highlight the finer points. same goes for jazz and wine tasting.