For me, it's all about the interplay/rapport between players when it comes to jazz. Love to hear a soloist work off of what the piano player is laying beneath and vice versa. The best drummers of 'em all (with the possible exception of Danny Carey) are jazz drummers. They get to do so much more than just lay down a backbeat. You can listen to practically any jazz tune, concentrate on JUST the drums, and walk away entertained.
I prefer at least one horn in any small ensemble jazz set. There are a few piano/bass/drums set-ups that I've heard that are decent (I've got 'em by Bob James & Duke Jordan, but then that Jordan set is blessed with the presence of Roy Haynes, one of jazz's finest drummers so it's bound to be worthy). Tenor sax is my preferred "4th Instrument" but if we're talkin' Miles Davis I'd just as soon change my vote to trumpet. Quintet is even better with tenor and trumpet.
Then again, there's nothing like the massive ensemble that Miles put together for the recording of Bitches Brew. I shan't go into a rant, as I have often proclaimed my love for this album. But there's something aboout the bizarre entrance of a bass clarinet into the, until then, traditional jazz instrumentation. But once it's in the mix it's incredible, making the funkiest lines ever composed on bass clarinet.
Bitches Brew is not your typical jazz album, not even your traditional fusion jazz album, as it is composed of edited sequences that have been re-pasted into different orders by the artist...but nevertheless deserves the accolades it has recieved throughout the years as the birth of fusion.
Speaking of which...when it comes to fusion, make no bones about it, it rarely gets much better than WEATHER REPORT. Consistantly they have impressed me from the days before Jaco Pastorius joined on bass, to the glory days, when his playing raised the bar for bassists everywhere, up until now, in the post-Pastorius days. Just pick something of theirs and you're guaranteed to love it.