For me the best tennis announcer right now is Courier (and ironically, I did not like him as a player). He offers good insights into what's going on during a match, in the locker rooms, on the practice courts, etc., but also knows when to shut up.
But most importantly, he isn't afraid to call out a player for a bad decision that went well, or a good decision that didn't. As an example, I'll routinely hear him say things such as "sure, he won the point, but that isn't the tactic they should be employing in this situation", or "I like that play...they may have lost the point, but it's the correct action given the situation, and down the line, it should pay dividends...".
I also enjoy Cahill, but they always pair him w/ one (or both) of the McEnroes, and sometimes Gilbert, three of the worst announcers IMO.