You're spoilt for choice for DMU sounds .
My '121 has Jamie Goodman sounds and it sounds great!
Legomanbiffo , Wheeltappers and Digitrains (Paul Chetter) also do excellent sounds that allow manual gear changing .
As to what is the best , we'll, that's for you to decide as sound is so subjective .
YouTube has clips of all the above to help you make your mind up .
My model had a sound chamber to enhance the sound from the speaker but you could do away with it and try one of the many speakers available these days , space permitting !
Hope this helps .