5 plus the transition
Wolfgang Amadeus Mozart wrote many Piano Sonatas, Concertos, Duets, Trios, etc. There are probably anywhere between 200-300 piano pieces written by Mozart.
Mozart wrote 18 complete keyboard sonatas for the "piano" of his time (which was not, strictly speaking, the modern pianoforte). There is also a traditional "19th" sonata which consist of two separate works usually performed as a complete sonata. You can find all 19 of them, chronologically, in both MIDI and MP3 form at: http://www.sdrodrian.com/mozart/sonatas/
See the Web Link to the left for some information. It is in PDF format.
Mozart's sonatas are actually among the least of his compositions. They are beautiful, but the concertos are more significant as piano music, and among his total output they are not among the most important works by any means.
Mozart wrote 41 symphonies. He also wrote 19 piano sonatas. His concertos include 27 for piano, 3 for flute, 1 for clarinet, oboe and bassoon each.
Mozart composed almost all genres of work of his time. He wrote 41 symphonies and 27 concerts for piano and orchestra. His other compositions include ensemble works and 19 complete piano sonatas.
Mozart's piano style ranges from Baroque counterpoint to Classical galant style. Most of his piano sonatas are written for saloon environments which contain occasional fugal movements. The best example for a composition in galant style is his Sonata for Two Pianos in D, K 448.
Mozart shifted music from the Baroque counterpoint to Classical galant form. For orchestra, he was one of the composers who developed the symphony and solo concerto. The other music genres emerged in his time are piano ensembles and solo piano sonatas.
Mozart wrote 18 piano sonatas, so you'll have to be more specific.
Beethoven and Mozart were both composers during the classical period. The most popular compositions in that period were symphonies and piano sonatas. However, music by Beethoven shows a huge contrast between dynamics, much more than Mozart's.
K 330 through K 333 are wonder sonatas, I would suggest all of them equally. Also, maybe the a minor (the number of which escapes me); though it might be overdone, it is likely the most virtuosic of his sonatas.
Wagner wrote 5 sonatas over his lifetime. He wrote two solo piano sonatas and one for four-hands during his studies and before his professional career. He wrote and published two piano sonatas in 1831 and 1832.