Which are the best journals for letters in mathematics (bordering with mathematical physics)?
As Pete L. Clark, I do not quite know what a letter is: short papers are usually quite similar to regular paper, only shorter and usually called notes.

Concerning the venue suitable for short articles, there are quite a few. First, you should now that most journal could accept short papers, but certainly you have more chance to get accepted in a journal specifically targeted toward short paper (except if you solve a known problem or reprove a notoriously difficult theorem, in which case no reviewer can use the length of the paper to argue it does not have much merit). Let me give my impressions on the journal that come to mind (to remind that this is from a biased perspective, I will often refer to the French mathematical community), limiting to journals that to not claim being restricted to a subfield -so called generalist journals.


CRAS (Comptes-Rendus de l'Académie des Sciences, Paris): this journal publishes both research announcements and complete short papers. The announcement part is less and less relevant, but I do not know whether it is less present now. Sadly, while CRAS published top-notch papers, it cannot afford to be very selective anymore. It also suffers from the research announcement role: French hiring committees often blankly dismiss CRAS papers, partially because they may be only announcements.
PNAS (Proceedings of the National Academy of Science, USA): this journal is not very well-known from French mathematicians, but publishes some quite good papers. I think it has a particular taste, but I cannot find the word to describe it. Beware that even if it has a page limit, the format is very dense so articles are not necessarily that short overall.
PAMS (Proceedings of the American Mathematical society): this journal is explicitly for short papers, but the page limit recently moved from 10 to 15 (AMS format). As far as I can judge it is selective, and sometimes publishes excellent papers. It is reasonably well regarded, benefiting from the sisterhood of Transactions AMS.
Bul. LMS (Bulletin of the London Mathematical society): this journal is the LMS counterpart to PAMS, and is somewhat similar (although a little less known and considered, maybe). Since the LMS series has three journals (Bulletin, Journal and Proceedings in increasing order of paper size) instead of two for the AMS, I guess that the average Bulletin paper is somewhat shorter than the average PAMS paper, but the different format makes it difficult to judge.
AMM (American Mathematical Monthly): this journal is well-known and extremely selective. It publishes papers of general interest, of varying length. The notes are typically a few pages long. It is even possible to publish mathematical facts of a few lines, in a grayed box. Since it is considered somewhat recreational, publishing in the Monthly gives less weight to a CV than its harsh selectivity would do for any other journal.
Elemente Math. (Elemente der Mathematik): this is a Swiss cousin to AMM, much less known and much less selective.
MRL (Mathematical Research Letters): this does barely belong to the category of note-publishing journals: it does not usually published paper more than 30 pages, but the average paper there is not a note as far as I can judge.