Microsoft generally doesn't recommend one over another as a matter of policy -
all the workflow  vendors are partners and they don't want to play favorites.

I m not frm Microfoft. My personal opinion is to use
k2net workflow  as of now I feel its better.

Im not sure the question was asking for a Microsoft recommendation
as much as a recommendation from any of the users who have had an
experience (good or bad) integrating a workflow  engine with
SharePoint.

We have just been given the "go ahead" to use NINTEX with our
project. We are in the early stages of a rather large effort. I
can't say one way or the other, yet, that NINTEX is the "be all and
end all" of workflow, but I can keep you apprised as to our findings.

I really haven't heard much about any other product.

Please try Datapolis Workbox. it is different then NINTEX because we have focused on state workflow  model (human-centric). More info: http://www.datapolis.com/en/workbox.html

Flash tutorial: http://www.datapolis.com/en/tutorial.html
Trial: http://www.datapolis.com/en/wb_freetrial.html