To add to the lack of consensus, how about Lanson Black Label Brut NV? It has managed to stay under $40 while my former favorites that used to be under $40 have crept over that bar the past year or so. Lanson is very decent, I think, but seems to have little cachet.
I can second Laherte Freres, Bonville, the Hebrart wines. Some of the others are hard to find under $40. With Delamotte, I prefer the Blanc de Blancs, but it’s over $40.
Also, there are style differences, really is no “best”.
at the JJbuckley private price I grabbed some heidsieck reserve brut for $36. that was a pretty great find if you happen to be on one of their representatives lists.
Charles Heidsieck is great buy under $40. It’s somewhat a more mature style since it has a larger percentage of older reserve wines (although I seem to recall a post from Brad Baker that the average age has been dropping as stocks dwindle).
Christian Busin is now owned and produced by Labruyere family:
My understanding is bottles labeled as Christian Busin are now produced only for long time, French customers. All other production is under Labruyere label. So, Busin bottles available in the US market now, or after 2012, are bottles procured through alternative channels, i.e. Grey Market, and not through an authorized, National importer.