I've tried a few and the only ones I use on a regular basis are custom howlers from the guys on here or a howler made by a local guy. The commercial howlers I've tried are the E.L.K., Red Desert, Sceery, Crittercall, Songdog, and Primos Hotdog. The custom calls are the way to go, besides, you can get most if not all of the custom call makers on the phone for some 1 on 1 instruction.
*Oh, if you are stuck on getting a commercial howler, the E.L.K. Power Howler sounds the most realistic, but lacks volume. The Red Desert, Sceery and CritterCall Magnum sound good and are loud. That said, The E.L.K. is plenty loud enough except in very high winds. Also, you'll all end up with one or more custom howlers sooner or later.