I own all of them except the UIVER DC-2.
All three are good aircraft

The Twin Otter is a classic. The VC is nice (although a little blurry depending on zoom levels), the engine sounds are great. It is getting older compared to what developers have done with newer add-ons.
The PBY Catalina has a beautiful VC, beautiful exterior model and liveries, a WWII classic, and is probably the one that has the most extensive systems models (out of the three) I would say. A cold dark start is a pretty cool thing to do and when those big radials start spitting black smoke it definitely puts a smile on my face!
The BN2 Islander is my favorite out of all three. The liveries are very nice, the aircraft is a slow small transport aircraft and flies like it. The VC looks great and actually shakes on takeoffs and landings! The sound package is the one I like the most (although all three have HIGH QUALITY sounds, so the Islander doesn't win by much). It probably is the best out of all 3 for bush flying.
I stick to it and say BN2 Islander!

Best thing would be to go to youtube and look at videos of all of them.
Good luck with your TOUGH decision and let us know what you got.