I love XM satellite radio!
On a recent trip to Dallas/Fort Worth, I was able to listen to virtually any major league baseball game being played and it certainly makes for an easier 3 1/2 hour drive.
San Francisco's Jon Miller is probably the best guy out there right now. He's fluid, easy, doesn't get too emotional into games or over-the-top with his calls (like the Yankees John Sterling).
Cincinnati's Marty Brennaman is also at the top of the list. His been doing it for more than 30 years, but he, like Miller, is very smooth in the booth.
One of the underrated guys out there is Milwaukee's Bob Uecker who calls one heck of a game. Very entertaining, but again, not over-the-top with his homerism.
My all-time favorite announcer is Ernie Harwell, who broadcast Detroit games for what seemed an eternity. His soft southern accent was the perfect complement to listening to a game. His mind was like an anecdotal encyclopedia. I remember as a kid him telling stories about games from the 1920s or 30s and casually talking about players through the golden age. He didn't shove statistics down your throat (like ESPN does), but kept the game interesting as if he was your grandfather. He has been sorely missed since he retired a few years back.