In my personal experience:
1. Quad ESL 57 Listening to a refurbished pair now, and they are the most
non fatiguing I have ever heard.
2. Proac Response 2.5 Very close second. Might be up for sale though!
3. Dynaudio C1 Signature Not quite as non fatiguing as the others, but
delivers incredible dynamics, and soundstage.
1. Quad ESL 57 Listening to a refurbished pair now, and they are the most
non fatiguing I have ever heard.
2. Proac Response 2.5 Very close second. Might be up for sale though!
3. Dynaudio C1 Signature Not quite as non fatiguing as the others, but
delivers incredible dynamics, and soundstage.