The best? Best is a hard question. Equipment wise, RM Young makes "scientific" grade equipment and is used extensively by NOAA and many universities.
Another contender that looks pretty good and has impressive specs would be Columbia Weather Systems.
However, these have their price. They ain't cheap.
After that, I would go with Davis Instruments and their various packages. They put out reasonably good data and are generally robust enough for what we do. Pricing is much better too.
Oregon Scientific would be the next one I would look at, but again, you get what you pay for.
Mounting to a vehicle is a home grown affair. Take a look at the NSSL Mobile Mesonets and the various Universities that have these. Also, do a search through the archives here and you will find as many mounting options as there are vehicles to mount.
Some things to consider before emabarking on this:
1. What are you going to do with the data you collect?
2. Do I want this permanent or do I want to remove it?
3. Do I really understand what's going to happen to this when it gets hit with a 90 mph gust?
4. Is it safe?
That probably doesn't help much but it will give you a start. A trick here is to do a search on the forum before asking too much. There's a wealth of information out there and there are many of your questions already answered.
Good luck!
