Every site has its limitations. There is no one place to go, and there never will be. Taxonomy and breeding change so rapidly that as soon as a list is "finished", it is out of date. They are still finding new plant species in China every week! Now multiply that by at least 50 for cultivars in the world, and you can understand how daunting such a task would be.
For instance, GRIN focuses on "lifeforms important for food and agricultural production", but also updates its Taxonomic Nomenclature Checker with data from other sources. Reprinted from the site:
"While GRIN includes virtually all currently accepted generic names (over 14,000), please note that the representation of species is incomplete, especially for non-agricultural plants and their relatives, so some of your names may have to be checked elsewhere."
I won't even get in to the ICBN versus the ICNCP.
