When using a program like TuxGuitar to write/transcribe songs, what instrument or instruments make a decent approximation of a vocal track for playback on a computer with an embedded soundcard?
midi sounds do tend to not quite sound like their namesakes. Honestly use a choir sound if you are writing for a choir use one of the choir sets, if it's melody then it's a little different. you'll not find anything that sounds like a lead singer in a band.
Instead think about
I'd usually use a flute, or one of the slightly harder-edged woodwind sounds. You could try the trumpet.
Although the sad truth about standard midi is this. You will not find a sound close to a person singing a song. There's a good chance that your midi melody will make it all sound like lift music, sorry.
No single MIDI patch, or instrumental sound generally for that matter, is going to be able to produce the breadth of sounds a singing voice can produce (whether singing vocalise or with lyrics). A MIDI patch will neither be able to match the range of timbres created by vowel sounds, or the range of articulations produced by consonants. Even passages sung without words can only crudely be imitated by using vocal "ooh", "lah" or "doo" sounds, for instance.
If, on the other hand, you simply want vocal lines to be easily heard amongst other instruments, I would suggest simply using something: with a clear attack; with some sustain; that is different enough in timbre to other instruments in any recording/transcription you are creating. It's also a good idea to use something common enough to be available on any machine. Personally, if you're not using piano already, I'd use a piano sound, not because it sounds like a voice, but because it fulfils the criteria above. However, if you have quite a lot of sustained, long notes in your vocal melody, a woodwind or brass sound may be more useful (as @AJFaraday suggests).
If I want to listen back to arrangements of songs I have created, I get very frustrated by the "doo-doo-doo" patches my software uses by default; I nearly always switch it to something like piano (or something like a synth-lead, if I'm feeling particularly perverse…)