Forrester Research, an independent market research firm, released in February 2014 the quarterly Forrester Wave Big Data Hadoop Solutions, Q1 2014 Report . The report shows this graphic, and it looks like that all major, minor and non-hadoop Vendors think they lead. It looks really funny when you follow the mainstream press news.
IBM  think they lead, Hortonworks  claim the leadership too, MapR  leads too, Teradata is the true leader (they say) . Cloudera 
ignores the report. The metapher is - all of the named companies are in
the leader area, but nobody leads.
Anyway, let us do a quick overview
about the "Big Three" - Cloudera, MapR, Hortonworks.
The 3 major Hadoop firms (Horton, MapR, Cloudera) are nearly in the same position. All distributions have the sweet piece, which lets the customer decide which one fits most. And that is the most important point - the customer wins. Not the marketing noise.
Cloudera  depends on Apache Hadoop, has Cloudera Manager, a strong, sophisticated and great tool to manage an entire hadoop cluster, including add, relocate and remove services from a node to another. In addition to the Open Source version of Hadoop they offer Closed Source Applications on top, like Cloudera Manager Enterprise, Cloudera Navigator (Data Lineage), BDR, Snapshotting, Data Replication. But these additional services aren't OpenSource.
MapR  is the most convenient guy here - the press release on their website is clear, no big noise. The message: Choose what is the best for your business. Makes the company a bit friendly. MapR has 3 different solutions - M3, the free-to-use edition, M5 - the Enterprise Edition with NFS Support, Snapshotting, independent code support and M7, the Enterprise Database Edition, optimized for Low Latency and High Throughput. MapR Editions aren't Open Source, and the management console is not as feature-rich as Cloudera Manager. Additionally, the company created their own HDFS-like file system (MapR-FS), mostly written in C(++).
Hortonworks  is the youngest player in the market. Originally Horton comes from Yahoo and is a spin-off from the core developers on Apache Hadoop MapReduce, Apache Hadoop HDFS and Apache Hadoop Yarn. HDP, the Hortonworks Edition of Apache Hadoop, is the only 100% Open Source distribution in the market. The managing tool, Apache Ambari (incubating) is also not so feature-rich as Cloudera Manager, but it's Open Source and works well. Furthermore, Horton sells only Apache Projects in their distribution, for Data Governance Falcon, and for Security Purposes Knox.
All of these three players have a strong support department as well as service delivery (Solution Architect), Pre- and Post Sales and a significant amount of customers.
In my eyes, I see only one true leader. Apache Hadoop. All of those "BigData" companies rely on a great idea, originally developed at Google and rebuilt by the Apache Open Source Community. This is what true leadership means - evolve and divide.