I have used all three.
If you mainly need video, which I think is the case for most, go for air video. The server is pretty stable, the login method over the web is smooth (though personally I don't use it often for reasons of security) and the playback is fine. It does place a pretty heavy load on your PC, so if you want to stream HD video, be prepared for 95% CPU load (on my 3GHz, Quad-core Intel).
Stream to me is very similar, the quality is practically identical and has the nice touch that it tries to decode only the streams necessary, i.e. anything the iPad can play natively (e.g. h264 in mp4, or aac in m4a containers) it just streams without decoding. This keeps your CPU use a bit lower, around 65% on my system. However the interface is not quite as polished and intuitive, some settings are in weird places and the server is not quite that stable (at least on windows).
The most important thing though is that it also streams music and pictures. This is great if you have no other apps that do that but for pictures, its not really that great because it takes a while to stream each picture so your user experience is slow and awkward.
There are better ways to make your picture library available online and much faster server apps if you just want to grab if from your PC.
For music its nice but again, it takes a while to start each song whereas other apps do it instantly.
Zumocast does several things I really don't like, like putting your files on a website which means your privacy is like gone forever and if you're careless you can delete the files on your PC from their website, which is an incredibly idiotic way to write an app. So I will just never use their stuff again, period.
In the end I prefer to have one app for each thing though I do miss the simplicity of Windows Media Player which keeps everything in one library and instantly streams all of it. If only there was a version for the ipad...
Pick air video for video, check out Spotify for streaming audio and any online picture library for your pics.