I consider Mackerras perhaps the most under-appreciated conductor ever. His recordings are consistently good-to-very good, and some of them are really quite insightful. An example of recordings by Mackerras that are different and interesting are his recordings of the Brahms symphonies with the Scottish Chamber Orchestra. It may not be a first choice, but it is so different that it is a good complement to whatever that first choice might be. His Rimsky-Korsakov Scheherazade is very good, with excellent audio quality to boot. His Rachmaninoff Symphony 3 is worth hearing. So, even though I've not heard his Mahler yet, it is no surprise that it is good.