What are the best hearing aids for musicians?

As others have indicated here, having instruments with the widest dynamic range is important, particularly in the input limit of the microphones.  Hearing aid microphones have a general input range of 115dB, but they can't always utilize the entire range depending on how they are otherwise utilizing other features in the instruments (i.e. noise reduction, speech enhancement, etc.).  The largest range I've seen is in the most recent Widex products (Dream, Unique, and Beyond), which have an input limit range of 113dB--almost the maximum possible.

The out-and-out expert on all this music-related for hearing loss is Dr. Marshall Chasin.  He routinely writes articles for Hearing Health & Technology Matters, and has his own practice in Canada.  He's my default source of information on hearing loss and musicians, and provides a great resource on the considerations of hearing aids for musicians.

In my experience because of their large dynamic range, proprietary fitting formula & option of a dedicated music mode or dedicated music streaming mode I would say Widex Unique & Beyond.

Recent reports I have received about the Oticon Opn 1 being used in club/concert type environments suggest they also work very well & when combined with their TV adapter (can be linked to other devices, not just TV's) can produce a clear high-quality stereo sound.

I hope this helps.

The short answer is none... nothing will reproduce the sound that is pure other than a music monitor which has the dynamic range of the normal ear.  That said, I would advise trying out a few brands and listen for the sound you feel is best.  Every brand sounds different, and people have preferences which they feel is "normal" ...  Widex Resound and Starkey seem to have excellent music programs.  I have worn all three and found music to be really smooth and not too pitchy.  I have not worn the OPN for music yet, so I cannot speak to its sound for music.  Be sure to have a special music program installed, be sure to turn off all noise suppression and feedback managers, as they will interfere with the sound of the music quality.  

The two hearing aids which have the widest bandwidth & dynamic range (and consequently the least distortion) currently available on the market are the Widex Beyond 440 and the Oticon OPN1.   Both use a large 10 kHz bandwidth, and dynamic range of 103+ dB.  The best sound will be available on the premium hearing aids, as bandwidth is diminished on the lesser models.  

This largely depends on if you are listening to music that is streamed directly into the hearing aids, or if you are listening to it live or through an external speaker.  If you are streaming the music to your hearing aids, your best bet is a device that can directly stream from an Apple device.  These devices include Oticon Opn, Widex Beyond, Starkey Halo, and Resound Linx/Enzo.  If you are using an intermediary device such as the Phonak ComPilot or Signia (Siemens) easyTek, the transfer of the stream through the additional device may cause some sound quality issues.  The problem with most hearing aids is that you're lucky if the sound can be reproduced anything above 6 kHz.  This means that you will not hear the music as intended by the artist.  The only hearing aid that can reproduce sound up to 10 kHz is the Earlens.  However, they are a new company and still going through hiccups with their technology reliability.  They are currently beta testing direct streaming through the iPhone which will improve the sound quality when complete. 

You can click the link HERE to see a comparison of traditional hearing aid bandwidth vs. Earlens.  Traditional hearing aids are just not capable of reproducing sound up to 10 kHz despite what they say or what the programming software says they can achieve.

Take a look at the Starkey Muse line of hearing aids which were created to do a good job reproducing music.  Most of today's top of the line hearing aids amplify out to 9,000 and 10,000 Hz.  These are the hearing aids I use when listening to music.  The Oticon Opn has been mentioned here and it, too, is a good sounding hearing aid.

The type of hearing aid (or brand of hearing aid) is not as important as making sure that the hearing aids come with a dedicated setting, specifically for enjoying music.  Most hearing aids have this capability.  One brand that has a great reputation for their Music setting is Starkey.