I love to play around with numbers and since I have been keeping track of sending everything out since 2009, I have kept pretty crazy stats on the successes and failure rates. I mostly send to Canadian NHL, AHL, and CHL teams with some retired players homes thrown in and the odd US team if I get my hands on some US stamps. So, here is the stats to date below.
2009, total sent 55, total received 39, percent 70.91% Top team...Calgary Flames 6/6
2010, total sent 62, total received 44, percent 70.97% Top team...Vancouver Canucks 4/4
2011, total sent 49, total received 36, percent 73.47% Top team...Calgary Flames/Ottawa Senators 4/4
2012, total sent 33, total received 23, percent 69.70% Top team...St. John's Ice Caps 4/4
2013, sent so far 50, received 22, percent 44% Top team...Hamilton Bulldogs 3/4
I have had really good luck with all of the Canadian AHL teams, so I suggest mailing to those teams for some great prospects.
As for the NHL, it is a bit of a mix. Calgary and Ottawa are really good. Vancouver is close behind. The Leafs, Oilers and Habs are middle of the road, and the Jets are absolutely terrible.