I recommend getting it with the company that has the underlying policies. In addition to the possibility of a multi-policy discount, it also means the coverage is "seamless". If your primary auto policy covers you for a rental car that you let your brother-in-law use and he handed to keys to a friend who wrecked it, your Umbrella probably covers that, too. In the case of a really big claim, they'll bring in the best legal minds because they're on the hook for not only the primary coverage but the Umbrella amount. It also means you don't have lawyers from Company A (which write the primary) and Company B (which wrote the umbrella) having to co-ordinate defense.
No specific company recommendations- just get one with a good rating from A.M. Best so you know they'll be around if you need them. The true test of an insurance company is how they handle claims and, hopefully, you'll never need to find that out.