Enterprise Resource Planning (ERP) software covers a variety of business processes from purchasing to accounting. The most used ERP software could be the most sold or the most implemented ERP software package applied to business processes; because of the difficulty in obtaining information about the most implemented and the depth of those implementations, most sold will be considered. Claims of ERP vendors are extremely suspect, as some vendors count modules sold or implemented versus entire ERP packages.
SAP is king of the hill; the reason is aggressive marketing and product development to back it up. SAP wins because their software has more features and areas covered; they have developed an enviable partner program that guarantees implementation expertise in any locale and around the world. Because of the number of features, SAP can take more time to implement; SAP seems to have solved this problem with development of aggressive "phase 1" implementations of a focused feature set followed by multiple implementation phases thereafter. SAP has clearly led the large company market for years; their foray into the small-to-medium business market with a software-as-a-service solution could lead to dominance of that market as well.
Oracle, unlike SAP, began as a database software company and has had a schizophrenic attitude towards ERP. They've leveraged their dominance in database technology to be the key competitor to SAP in the ERP software market. Oracle utilizes a strong partnership channel but also has healthy implementation services resources; however, Oracle's lack of focus on partner-led implementation, and the mix of database and ERP software, have contributed to their second-place ERP software status.
Although NetSuite does not get close to SAP or Oracle in terms of ERP software sales, they are the king of ERP in a different and growing arena: software-as-a-service (SaaS). SaaS allows use of software over the Internet thereby eliminating hardware on-site. NetSuite was born out of investment by Oracle but remains an independent company. SAP has chosen to attack the SaaS market internally; SaaS is the future of ERP, but the timing of that future is uncertain and subject to perceived SaaS weaknesses in the data security and internal control-of-systems areas. NetSuite is targeted to the small-to-medium sized business market, an area which SAP and Oracle have attacked and achieved mixed results so far.
Infor is an ERP software company with a completely different business model. Although they arguably occupy third place behind SAP and Oracle in terms of ERP software sales, their growth is through acquisition. Having acquired 12-13 mid-market ERP software solutions, they are highly dependent on sales of ERP add-on solutions to their customer base. Their business model makes it difficult to compare them to competitors and has proven to be difficult to understand by investors.