As transparent and as high in resolution of details your budget can afford. Please be patient with my explanation.
No speaker is best for Mahler's music unless you understand what Mahler was trying to communicate with his music. Start by attending the live performance of a Mahler symphony. If that was not feasible, listen to Mahler's symphonies No. 3, 4, 5, 6 and 9 conducted by Benjamin Zander on the Telarc label. Each of these CDs contains a separate CD with the conductor discussing the background and orchestration of the symphony to help you understand Mahler's music in case you are not already a Mahlerian scholar.
I am no Mahlerian scholar; however, after doing the above, I realized that the essence of Mahler's symphonies lies not only in the contrast and dynamics but also in the complexity weaved by the different instruments that play different themes. Hearing everything is thus a requirement, at least for me; and to accomplish this, a system (not just speakers) must yield high transparency and detail resolution as a top priority. Transparency and resolution usually brings along other audiophile qualities.
Not knowing your budget and not having heard all the speakers in the world, I can only recommend ones that I have experienced myself. I prefer speakers that have a higher efficiency ie. 91db and above so you do not have to spend too high a price for high power amplification. Needless to say, an great amp with a lot of current also helps push out the details. Also, take advantage of the SACD format.
I recommend a used pair of Wilson Watt Puppy of versions 3.2, 5.1 or above, or the B&W Nautilus 803 and above. All are efficient, but the B&Ws like more power than the Wilsons. Buy these used and save a lot of money.
My system is as follows as an example:
Arcam CD23T (can use a nice SACD player)
Krell KRC-3 preamp
Mark Levinson 331 power amp (can use more power here)
Wilson Watt 3 Puppy 2
Transparent Super XL interconnects and speaker cables
I have attended live performance of Mahler's symphony's no. 2, 3, 6.
Hope I helped. Feel free to contact me outside the forum.