Windows 8 and
Windows 10 integrates a more robust version of
Windows Defender (and uses that name) for its
anti-virus and
anti-malware protection. Windows 8/10 Defender provides the
same level of protection against malware as Microsoft Security Essentials (MSE) provides on older operation systems and uses the same daily virus definition updates.
Although it uses the same name, Windows Defender 8/10 is
not the same as Defender in previous operating systems like Windows 7 and Vista which was only an
anti-malware (anti-spyware) program intended to supplement your anti-virus (not replace it).
IMO
Windows 8/10 Defender is just as good as any other free antivirus solution (and probably easier to use for the novice) without bundled toolbars or nagging popups.
Some anti-virus vendors will ask you to remove Malwarebytes and other security software prior to installation in order to avoid conflicts when the reality is they only want you to use and rely on their software. Conflicts typically occur when installing two anti-virus products but Malwarebytes Anti-Malware Premium is
not an anti-virus program...see
Does Malwarebytes Anti-Malware replace antivirus software?Malwarebytes Anti-Malware Premium works well and should run alongside anti-virus software without conflicts. In fact, Malwarebytes has been conducting AV Compatibility Testing for years.In some rare instances, you may need to add
exclusions for both the anti-virus and Malwarebytes in their respective exclusions list to resolve problems and achieve the best possible system performance.

There are members of the Microsoft community who recommend only to use the free version of Malwarebytes Anti-Malware as a stand-alone scanner to avoid possible conflicts. Although there have been some reports from Windows Defender users that it will not work when Malwarebytes Premium is installed, others have indicated the two products work fine together so you may need to experiment and add
exclusions.
With all that said, you may want to read about the newest version of Malwarebytes which should be out of beta soon.
Announcing Beta of Malwarebytes 3.0, a Next-Generation Antivirus ReplacementMalwarebytes 3.0 Beta Forum