I don't know about "best" letter, but here's one of my letters, I think the last published in comics, that appeared in Walt Simonson's ORION 8, cover-dated Jan 2001.

https://comiconlinefree.com/orion/issue-8/22

It's the only one where I'm credited as Dave the Wonder Boy. They pulled it from a longer comment I left about the series on the DC boards.
I praised the first ORION issue as a perfect expansion of the biblical/spiritual warfare for Earth between the good and evil New Gods, that I felt began as one of the best expansions of Kirby's original series. Only the 6-issue FOREVER PEOPLE series by DeMatteis and Paris Cullins ever explored the spiritual warfare concept as intelligently.
But in the DC boards comment, I expressed that Simonson's run had increasingly declined from that high point ever since. The issue I commented on, issue 5 (which you can read at the same link) is an all-visual no-captions issue. In his response on the DC boards, Simonson openly said my call for continuing the more high concept themes he'd done so well in the first issue, that he "just didn't want to work that hard". And that shows in the lack of depth in the remainder of the run. In which Simonson largely farmed out a number of backup pages each issue to be drawn and written by other creators. But hey, it was nice of him to personally respond on the boards. Aside from the universaly reviled Mike S. Miller, Simonson is the only DC creator I can recall responding to criticism on the DC boards (at least under his own name). And hey, at least he excerpted one of my online comments on the letters page.

It's the only DC boards comment by me that ended up printed in a letter (that I'm aware of). Although I detailed on the DC boards in a topic how I wanted a set of Neal Adams Batman hardcovers to be presented, broken into three volumes. One for BRAVE AND THE BOLD, one for Adams' DETECTIVE issues, and one for Adams' BATMAN issues, with all the BATMAN, DETECTIVE, B & B and WORLD'S FINEST covers inserted chronologically in between the stories across all three volumes. DC pretty much followed my concept in the BATMAN ILLUSTRATED BY NEAL ADAMS hardcovers. Except that Adams himself screwed the hardcovers up by re-drawing large portions of the books.
I'd like to see the Adams stuff reprinted again, but this time without the alterations by Adams, but as they originally appeared.

Possibly other things I said had influence that I'm either unaware of or have forgotten.