The honest answer to your question is, it depends a lot on what you mean by "EMR/EHR" software products (or providers), since there is a wide range in the market in terms of functional scope. It's a crowded market too - most lists you'll find on the web have more than 200 names on them; I like KLAS Research's listings (<a href="http://www.klasresearch.com">www.KLASresearch.com</a>) because they allow you to search by software product or vendor or professional service provider, and also to filter based on whether they have any rating data on the company in question, but even KLAS, which has ratings on over 250 companies, isn't comprehensive, since it doesn't include open-source vendors like MedSphere and DSS that sell packages offerings of VistA.
With those disclaimers aside, the names often seen on "top EMR" vendor lists include Allscripts, athenaHealth, Cerner, CPSI, eClinicalWorks, EPIC, GE, McKesson, Next Gen, Praxis, Quadramed, and Siemens. Frequency of citation is by no means the same as an endorsement, but you might notice that there are both very large and very small companies out there with claims to being "leading" vendors. Bear in mind also that most ranked lists are based on product or company revenue, rather than number of implementations or functional scope, but functionality is often the most important aspect for potential EMR/EHR buyers to consider. As another data point on the market, the <a href="http://www.cchit.org">CCHIT website</a> lists 177 EHR products certified against criteria for Ambulatory Care, 15 of which are preliminarily certified against the 2011 meaningful use certification criteria.