What isn't clear is what exactly this implied beyond "best." Thomas Frognall Dibdin writes (here), of different editions, that "the chart, opt. is barely as good as the common paper of the Latin Bipont classics" (93), “The charta optima is only a better sort of charta cacata” (297) and “The charta optima is little better than ballad paper” (511).
Having looked into this a little further—with the help of Jane Wickenden, Boris Bruton, Donald W. Krummel and John P. Chalmers from the EXLIBRIS list—I have compiled a very short list of some of the Latin abbreviations used concerning different types/grades of paper. I am publishing it here in the hope that, if anyone else on the planet were to Google "Ch. opt."—they will find the answer to their query on this page.
Charta optima [Ch. opt. or chart. opt.] = best paper
Charta maxima [Ch. max. or chart. max.] = large paper
Charta cacata = crappy paper or toilet paper
Charta scriptoria = fine paper
Charta velina = vellum
Charta membranacea = vellum
* * * * *
Charta optima is glossed as papier supérieur [superior paper] here.
The British Magazine 30 (1 October 1846): 362: “The small-paper leaf measures eleven inches and three-eighths by seven; and many of the leaves are marked with an obelus †, which is not seen in either the large paper or the charta maxima, nor can I explain the cause of this variation.”
Max Brod, Tycho Brahe's Path to God (2007), 92: “Perhaps that would have been better; for, if I may say so, the book is a collection of refuse, a charta cacata.”; Joseph William Moss, Manual of Classical Bibliography (1827), 1.205: “There are copies on large paper, which are very rare and beautiful; those on common paper cut a very indifferent figure; the paper is extremely bad, and, to use Dr. Harwood’s words, truly is ‘Charta cacata’”; Thomas Fuller, The Church History of Britain (1837), 159: “Nor stops he here, but useth so slovenly an expression, (it is well it is in Latin,) calling his book Charta cacata, which, saving reverence to the reader may be returned on the foul mouth of him who first uttered it”; George Richard Crooks et al., A new Latin-English school-lexicon (1861), 131 sv Cado: "to emit from one's self at stool, odorem: … to besmear with excrement, charta cacata”
Moss, Manual of Classical Bibliography (1827), 1.[ix]: “It has not, in general, been deemed necessary to cite the varieties of fine and vellum paper copies, because it may be taken as a rule, that all the principal German editions are printed on three kinds of paper, of which the second in quality, called charta scriptoria, is usually published at a third or fourth more than the common, and the best or charta velina (i. e. membranacea,) at about double.”