Remember that the genre provides a broad classification, not an academically-rigorous description of a work's musical form. Thus ricercar, canzona, madrigal are not musical genres (in this context, at least), but are all manifestations of “Renaissance” music. But given that canzonas and madrigals were being written in the 18th, 19th and 20th centuries, even “Renaissance” is too specific to be a really appropriate musical genre. Remember, too, that the genre a work is classified as being part of determines its physical location on hard disk. That is, the physical storage hierarchy of digital classical music should be Composer → Genre → Work, and thus 'genre' should be regarded as a way to conveniently separate the output of composers into distinct, but broad, groupings.
With that in mind, here are my suggested genres for classical music (in alphabetical order):
(Optional genres in italics).
By definition, a ballet score is going to be a large orchestral work. You could therefore reasonably classify the genre of Britten's The Prince of the Pagodas or Tchaikovsky's Sleeping Beauty as “orchestral”. However, doing so ignores the distinctively dramatic, narrative nature of a ballet score. In general, I think ballets are long enough and distinctive enough to warrant their own genre category.
Bach cantatas are difficult to categorise as anything other than their own unique genre. Yes, they contain choral parts; but they also contain instrumental and solo vocal movements -so they are really not 'vocal', 'instrumental' or 'choral', but are entirely sui generis. Besides, there are so many Bach cantatas that seperating them out into their own sub-folder is a really useful way of organising the physical storage of your digital music files.
Small instrumental works with one instrument per part and intended for performance in a domestic or small-room setting. String quartets and quintets are self-evidently examples of this genre, along with flute, violin and cello sonatas.
Large-scale works that, whilst an orchestra may be present, are principally concerned with a choir performance. Requiem masses are a good example of this genre: Verdi's or Berlioz requires a huge orchestra, for example -but they are clearly more choral in nature than purely orchestral. Note that Oratorios are examples of choral works and could well be classified as such. However, oratorios tend to contain large parts for solo recitiative (the Evangelist, for example), and thus are not 'purely' choral in nature. I therefore regard oratorios to belong in their own genre, whereas I don't think requiems and masses in general are sufficiently distinct in nature.
Large-scale orchestral works that contain a part for a solo (sometimes two or more) virtusoso instrumental parts. Obvious examples are piano concertos, violin concertos and the Britten Cello Symphony (whatever the composer thought!). Yes, concertos involve large orchestras, but they are distinctive enough in character to warrant their own genre classification.
A number of 20th century composers wrote signficant orchestral works as radio plays and film scores. Often they are purely orchestral in nature and could, therefore, be classified as 'orchestral'. But they usually have a programmatic and narrative nature which makes them a distinct genre in their own right. Frequently, too, radio scores will contain spoken or sung narrative, so that they are not 'purely' orchestral or vocal works in any case.
I don't classify much music in this genre, but there are some works written for large-scale public performance (therefore, not chamber), but only for a handful of instruments (therefore, not orchestral). For example, Britten's Fanfare for St. Edmundsbury, which requires four trumpeters to perform in the open air.
A broad genre, covering everything from Harpsichord suites to Organ postludes, with piano études swept up on the way. There is a reasonable argument to be made for separating out harpsichord, piano and organ into their own, unique genres. But I didn't want to get into a fight with myself about whether a fortepiano should be classed as 'piano', or whether it was OK to include 'clavichord' under 'harpsichord'. Separating the genres also meant that performances of Bach's English Suite would end up in two different disk locations, depending on whether they were 'authentic' performances on the harpsichord or 'modern' performances on pianos. In the end, I decided sweeping everything into a single 'keyboard' genre was sufficient.
Covering everything from traditional 'Grand Opera' to one-act comic operettas, this genre is designed to group anything consisting of vocal soloists, chorus and orchestra performing works with an explicitly narrative, dramatic form involving (in real life) stage acting. Fans of Wagner will object that he didn't write operas, but “music dramas” or even “Gesamtkunstwerk” -but I think this is an example of erudition for the sake of it! Since Wagner is dead, I think we can safely regard his works as 'opera' for the purpose of classification and physical segregation on disk.
This is an optional genre, intended to separate out the usually-multi-movement-but-coherently-designed, large-scale orchestral work from other large-scale orchestral works that are perhaps in the form of a single, extended movement. Richard Strauss' tone poems, for examples, are dramatic, large-scale and purely orchestral -but they clearly aren't symphonies. In the end, for my own collection, I have classified all symphonies purely as “orchestral” works as I found I rarely needed to distinguish a 'true symphony' from any other big-orchestra work I might fancy listening to.
Intended to classify anything where (usually) a single soloist sings songs with piano or small orchestral accompaniment. Britten's song cycles, for example; Or Schubert's Lieder. The term 'small scale orchestral accompaniment' is not to be taken literally, either: Strauss' four last songs do not require a small orchestra at all, but I nevertheless regard them primarily as vehicles for the vocal soloist's artistic expression… and thus as 'vocal' works.
Most of these genres are adjectival in form. That is, we use “symphonic” or “orchestral” rather than “symphony” or “orchestra”. It's also “vocal” not “voice”, and “choral” not “choir” or “chorus”. That's because we are defining a collection of things plural, not naming specific instances of things. Thus, Beethoven's 5th is a symphony, but is of the symphonic genre.
This 'rule' of the adjectival is clearly not unbreakable: if we were to apply it rigorously then we should talk about the 'balletic' or 'concertal', I suppose! But without inventing a whole new vocabulary of unfamiliar (and thus unhelpful) words, I think the above list usefully balances utility and consistency.