I believe Bill Brewer (AlteredCart.com) sets a good standard for 3rd party module development & support. My own experience, and the experiences of customers I've read in the forums, have always been positive when it comes to Bill. Plus the fact that all of his products are well designed, fully thought out, and are continually developed.
I personally don't know of anyone else that stacks up to Bill's standard, myself included, but I've also not dealt with all 3rd party module developers yet.
Regarding 3rd party companies that aren't necessarily module developers, I believe Paul Dodman (company: LuminoInternet) and Padraic Ryan (aka balinor, company: RyanDesignStudio) are the 2 best 3rd party companies involved in the X-Cart community, which provide design and/or general services.
Between Bill, Paul, and Padraic, you can't go wrong
