For a glasspack, my favorite is Cherry Bomb! Quite simply because they are the original glasspack guys, and the red finish holds up fantastic even through the harsh road conditions we see every winter here in Minnesota! And you can't beat the legendary sound!
As for what the "loudest" glasspack is, there's a lot of variables that come into play. But as a general rule of thumb for glasspacks, the shorter the case length; the louder it will be. (Eg. A 12" glasspack will be much louder then a 20" glasspack) And the larger the ID of the muffler, the deeper the sound will be. Eg. A 3" ID glasspack will have a deeper sound then a 2.5" glasspack, the 2.5" will have a raspier sound to it.