Your best post evarI think they were both great orchestrators. Personally I think orchestration can be viewed in 2 ways: from an artistic sense and a practical sense. In the practical sense, I think it is quite possible to narrow down somebody as objectively better. Stravinsky and Ravel were way better at this than Beethoven or Zappa, in terms of writing extremely idiomatically for the instruments, being able to write so that everything is balanced and easily heard, things aren't covering up other things unintentionally, instruments aren't expected to play things that are extremely difficult for them without preparation, parts are divided up cleverly to allow players a chance to breath and rest. However, in the artistic sense, it all somewhat comes down to personal preference. In addition to the practical aspects, orchestration is about color. Timbre is extremely important, even going back to the classical and baroque and ancient music. Why have such a variety of instruments, if not for the colors that such variety lends to music? I tend to prefer Romantic through contemporary orchestration, because you have alot more imagination employed in it. In the classical period, everything is so homogenized, which is why I tend to not enjoy that period, so much is by-the-book in melody, harmony, form, and in orchestration. Personally its hard for me to choose a favorite orchestrator (just as it is hard to choose a favorite composer). In particular, though, I love Tchaikovsky, who I think it academically under-rated in pretty much every aspect of his writing (everybody favors either Wagner or Brahms). Brahms also did beautiful things with color in his orchestrations, though I think Tchaikovsky's is a bit more imaginative simply because his writing drew on more diverse influences, including his native Russian folk music, and the folk and classical music of western Europe. Both he and Brahms took heavy influence from Mozart and Beethoven in the classicist elements of their work, but Brahms' writing I think is stuck in Germany, and Germany I don't find to be especially interesting compared to other places.
tldr: I prefer Tchaikovsky, more diverse and imaginative in his writing than Brahms, imo. But I think they are both brilliant.