TTDPatch and OpenTTD are two different projects. They are both sequels to the original game. Between the two there are quite a few similarities, but also quite a few differences. Whatever one should consider as 'best' is up to personal taste.
For both games, there are 'stable' versions and 'nightly' versions. The stable versions are considered to be without problems, but the nightly versions usually contain extra features. In both cases I'd recommend the nightly versions. They're usually as stable as the 'stable' releases but without the nightlies you'd miss out on the extra features.
You can get the latest TTDPatch here:
http://nightly.ttdpatch.net/latest Also don't forget to get the graphics
here.
And OpenTTD:
http://nightly.openttd.org/
Both run on vista.
And you don't need to install any patches. Just make sure to read the manual!
TTDPatch:
http://wiki.ttdpatch.net/
OpenTTD:
http://wiki.openttd.org/