Composers

"O lieber Papa! Sie sind sehr gütig, aber es ist doch noch lange keine Schöpfung"
'Oh dear dad! You are very kind, but it is far from being a creation'
Inscribed. Allegedly to Haydn in 1801 after he praised Beethoven's ballet.   
Kerst, Friedrich: Die Erinnerungen an Beethoven, collected and edited by Friedrich Kerst, vol. 1, Stuttgart: Julius Hoffmann, 1913, p. 109

"wie ich neulich zu einem guten Freunde von mir komme, und er mir den Betrag von dem, was für die Tochter des unsterblichen Gott's der Harmonie gesammlet worden, zeigt, [...]"
'as I recently came to a good friend of mine, and he showed me the amount of what had been collected for the daughter of the immortal God of Harmony, [...]'
Beethoven to Breitkopf & Härtel in Leipzig, Vienna, 22 April 1801 (BGA 59).
Beethoven refers to a collection in favour of Regina Susanna Bach (1742-1809), the youngest daughter of Johann Sebastian Bach, which had been initiated by an appeal by Friedrich Rochlitz in Intelligenz-Blatt No. 13 zur AMZ 2 of May 1800, p. 56.  
Beethoven-Haus Bonn, Collection H. C. Bodmer, HCB BBr 7

"schreiben sie mir geschwind wie das am besten möglich sey, damit es geschehe, ehe unß diese Bach stirbt, oder ehe dieser Bach austroknet und wir ihn nicht mehr tränken können -"
'write me quickly how this can best be done so that it happens before this Bach (brook) dies, or before this Bach (brook) dries up and we can no longer water it -'
Beethoven to Breitkopf & Härtel in Leipzig, Vienna, 22 April 1801 (BGA 59).  
Beethoven had declared that he would publish one of his latest works for the benefit of Regina Susanna Bach, daughter of Johann Sebastian Bach, with the Breitkopf-Härtel publishing house so that she could benefit from it; however, Beethoven's plan was not realised.  
Beethoven-Haus Bonn, Collection H. C. Bodmer, HCB BBr 7

"ich behaupte fest, nur Mozart könne sich selbst vom Klavier auf andere Instrumente übersetzen, sowie Haydn auch - und ohne mich an beide große Männer anschließen zu wollen, behaupte ich es von meinen Klaviersonaten auch,"
'I firmly maintain that only Mozart could translate himself from the piano to other instruments, as well as Haydn - and without wishing to associate myself with either of these great men, I claim the same for my piano sonatas.'
Beethoven to Breitkopf & Härtel in Leipzig (fragment), Vienna, 13 July 1802 (BGA 97).
Original not known; text according to Thayer II, p. 183. Thayer had a now lost copy of the letter from Otto Jahn's estate, from which he published only the present passage. 

"Nicht entreiße Händel, Haydn, Mozart ihren Lorbeerkranz; ihnen gehört er zu, mir noch nicht."
'Do not snatch the laurel wreath from Handel, Haydn and Mozart; it belongs to them, not yet to me.'
Beethoven to Emilie M. in Hamburg, Töplitz, 17 July 1812 (BGA 585). 
Original not known; text after the first print: Thayer III, p. 205, where the source is: 'Communicated by Mr Matthias Sirk from Gratz in Styria'.

"für Haendel ein eigenes Gestelle – Dieß Gestelle muß bey jeder Lücke eine Leiste haben, welche leicht beweglich zu machen so für größere und kleinere Bücher."
'A separate rack for Haendel - This rack must have a ledge at each gap, which can be made easily movable for larger and smaller books.'
In his diary, 1812-1818.
Although written in Latin instead of German letters, it is not entirely certain whether Beethoven actually meant the composer by 'Haendel' here or whether he was referring to business papers.
Beethoven's Diary: 1812-1818, ed. by Maynard Solomon, Bonn: Beethoven-Haus, 2nd edition, 2005, p. 99  

"Genie hat doch nur unter ihnen der Deutsche Händel u. Seb. Bach gehabt,"
'Only the Germans Handel and Seb. Bach had genius among them'
Beethoven to Archduke Rudolph in Baden, Mödling, 29 July 1819 (BGA 1318). 
Vienna, Gesellschaft der Musikfreunde, A 84/112; cf. also Thayer, Alexander Wheelock: Ludwig van Beethoven's Leben. Volume 4, Berlin, 1872, p. 168

"Mozart's grösstes Werk bleibt die Zauberflöte; denn hier erst zeigte er sich als deutscher Meister."
'Mozart's greatest work remains The Magic Flute, for it was here that he first showed himself to be a German master.'
Attributed to. Allegedly in 1820 to the musician Ignaz von Seyfried.
Ludwig van Beethoven's Studien im Generalbass, Contrapunkt und in der Compositionslehre: mit Beethoven's Portrait und Facsimile nebst anderen artistischen Beilagen, aus dessen handschriftlichem Nachlassen gesammelt und hrsg. von Ignaz von Seyfried, zweite revidirte und im Text vervollständigte Ausgabe, von Henry Hugo Pierson, (Edgar Mannsfeldt), Leipzig; Hamburg; New York, NY: Schuberth, 1853, Anhang p. 21.

"Händel ist der größte Componist, der je gelebt hat."
'Handel is the greatest composer who ever lived.'
Attributed. A letter from the summer of 1823 about this alleged statement by Beethoven. Tyson identifies the author as Johann Reinhold Schultz. He contradicts Friedrich Kerst, who names Edward Schulz as the author of this letter (see Die Erinnerungen an Beethoven, vol. 2, p. 58).
Schultz, Johann Reinhold: A day with Beethoven. Extract from a letter from Vienna to a friend in London, in: The Harmonicon, vol. 2, no. 13 of January 1824, p. 11; quoted from Thayer, Alexander Wheelock: Ludwig van Beethoven's Life, vol. 4, Berlin, 1879, p. 457

"allzeit habe ich mich zu den Größten Verehrern Mozarts gerechnet, u. werde es bis zum lezten LebensHauch"
'I have always counted myself among the greatest admirers of Mozart, and will do so until the last breath of my life'
Beethoven to Abbé Maximilian Stadler, Vienna, 6 February 1827 (BGA 2113).  
Prag, Památník Národního Písemnictví  (Memorial of National Writing)

"er räumte ein, […] nicht Bach, sondern Meer sollte er heißen, wegen seines unendlichen unausschöpfbaren Reichthums von Toncombinationen und Harmonien. Bach sei das Ideal des Organisten"
'His name ought not to be Bach (brook) but Ocean, because of his infinite an inexhaustible wealth of combinations an harmonies. He was the ideal of an organist'
Attributed. Karl Gottlieb Freudenberg tells of his acquaintance and a conversation with Beethoven.  
Freudenberg, Karl Gottlieb: Erinnerungen aus dem Leben eines alten Organisten, ed. Wilhelm Viol, Breslau: Leuckart 1870, pp. 39-44; cf. also Thayer, Alexander Wheelock: Ludwig van Beethoven's Life, vol. 5, Berlin, 1879, p. 224