I like to have a commentator that understands the sport at the top level, be it football, cricket, rugby or whatever. So that means it is invariably an ex player who is able to give a good insight into what is happening, the mindset of the players, the tactics etc etc. Now try marrying that up with the ability to actually commentate in a clear concise manner and there are not many that I really like.
I suppose that is why they often team up ex players with commentators so they can do both sides of it.
Cricket is an example of where I love to hear real top players relate to what is going on out in the middle. The views of someone like Boycott who actually faced (without a helmet !!!) the top Aussie and West Indies pace bowlers like Lillee, Thompson, Holding, Roberts etc describing what it is like out there and how best to play fast bowling is a real eye opener and great listening.
I cannot stand commentators who think they have to shout and screech in a high pitch to try and make it sound exciting Pearce and a few of the ladies are a bit like that. Just describe what is happening don't get hysterical.