Android 
1. BlueStacks: This is one of the most popular Android emulators I'm using on my Windows machine too. It has a nice interface and the installation process is quite simple. Though it is aimed towards mobile gaming enthusiasts, one can use it as a standard Android instance and can install and use other apps too.
This emulator is cross-platform compatible and can run on Mac too. It fully supports both HTML5 and Flash making it an ideal choice for running rich Android apps using both the technologies under the hood. Its multitasking support allows you to run several apps in parallel for a true Android experience. The system requirements for this emulator are also minimal ensuring your dated PC can run it easily.
2. Andy: It is a powerful and feature-packed Android emulator capable of running both on Windows and Mac machines. It can easily access the native file storage system of the PC if required. Push notification support ensures you're informed about the updates in real time. The pro version can also run on Linux.
It can also connect and sync communication with a regular phone device so that you can easily exchange information between a PC and a smartphone. Like other professional grade emulators, this one also supports access to the local file system which literally gives you unlimited storage. It can also access PC's webcam and microphone to ensure you can use messaging apps that use both the peripheral devices.
3. YouWave: This lightweight and flexible Android emulator can run on all the popular Windows versions smoothly. It supports access to SD cards which allows data saving right from the emulator environment. One can easily run multi-player online Android games through this excellent emulator.
It has an excellent support for rotation detection and accordingly changes the app window to give you a real smartphone experience. Handy controls for volume ensure you can tweak the voice level on an app level. It gives full access to the play store so that you can install and run thousands of apps available online. It is one of the few Android emulators which are blazing fast and light on system resources.
4. MEmu: If you're a die-hard Android gamer, I'll highly recommend using this Android emulator. It has one of the nicest and cleanest interfaces among all the available options. Its performance beats even some of the high-end Android smartphone brands making it an ideal choice for all the enthusiastic users.
It can run on various hardware platforms and Windows versions. Users can also use a keyboard, a mouse, and a joystick to play an Android game through this emulator on a large screen. One of its striking features is the ability to create multiple instances of isolated and functional Android emulators with a click of a button. Its standalone installer is much smaller in size than other Android emulators.
5. KOPLAYER: This free and promising Android emulator can be used to run both gaming and regular apps from the play store. It supports both touchpads and gaming joysticks along with hardware acceleration if available. Multiple Google accounts can be used with this emulator without any issues.
It also has video recording support which enables you to save your session to be viewed later on a media player. Its adaptive resolution gives you the crispest interface on different screen resolutions. Both 32-bit and 64-bit variants of Windows OS can run it smoothly. Similarly, a wide range of CPU models is supported by different vendors based on x86 architecture. Its footprint is also small in the system.
6. AMIDuOS: This professional Android emulator is a must try which gives you unique features to get a full-fledged experience of this popular mobile operating system. Apart from running it on a regular x86 Windows machine, one can also run this powerful emulator on devices running on ARM v7 processors.
It supports hardware accelerated 3D rendering making it ideal for playing high-end Android games. This emulator can easily access popular peripheral devices likes a webcam, a joystick, and a microphone. Sensory controls enable you to experience a true device-like experience. It also supports full multitouch and screen gestures to easily navigate and use the apps. It also has a compass with GPS functionality.
7. Nox App Player: This blazing fast Android emulator has a customized kernel for added performance. It can run both on Intel and AMD processors. It also allows you to create custom key combinations while playing games. Custom UI skins can also be created for a personalized experience within an emulator.
Another unique feature of this emulator is the ability to convert a classic Android game into its PC equivalent. Its default interface is minimal without any clutter enabling you to easily focus on the settings and app management. I've also tested it on a low-end machine and found its working smooth without any lag. You can also use the Google play store to easily install regular applications and games.
8. Droid4X: This Android simulator is worth mentioning in this list. It can run almost every Android app without any issues. The interface is simple with no-nonsense controls. It can run both on x86 and ARM architecture with support for wide range of hardware configs. I've used it both on a laptop and a desktop.
An external controller module is also available that lets you control the simulator instance wirelessly from your smartphone. It can also run on iOS platform giving you an extra option to test on. Its fine-tuned kernel and graphics rendering module enables you to run apps at blazing fast speeds. It has full multi-touch support which enables users to play high-end games with ease right on their desktop.
9. Genymotion: Professionals who want to test and debug their apps through a fully functional Android emulator can use this solution. It comes with loads of features making it an all-on-one emulator which gives a true device-like experience. One can fine-tune vital parameters while using it for app testing.
The emulator also comes with several useful plugins which can be used to add or enhance features. It also has retina support to test the apps on a high-resolution screen. The emulator can be switched to thousands of different configurations giving you different device environments for a professional-grade testing platform. Power users can also use all the standard Android API libraries for the applications.
10. Android Studio: Though this is an integrated development environment aimed for hardcore Android developers, its emulator can be used by savvy and geeky users for running their applications and games. The IDE comes in a bundled format and can be installed without any complex setup within a minute.
Its emulator can be configured to simulate almost any Android device to give you a perfect virtual environment to run and test the app. The emulator itself boots very fast and supports all the vital sensor features to give a real Android experience. One can also resize the emulator screen dynamically to see how the apps adapt to the change. Predefined device configurations are available within the emulator.
Android as a Desktop OS for a PC
There's also a full-fledged desktop variant of an Android operating system which can entirely replace your existing Windows OS and can give you an experience of a regular desktop environment. You can install Remix OS on our existing PC and can get a taste of this promising Android desktop OS.
Technically sound users who're reluctant to try it by replacing Windows altogether can use its VM image which they can install on VirtualBox. This way you can easily test all the features of this Android desktop OS within an isolated container keeping your existing OS intact.
The latest version of this Android OS can also be installed beside your existing Windows instance giving you a regular dual-boot device. I've tested this excellent OS on an HP laptop and found it very fast and user-friendly. It has a stunning interface with features taken from both Mac OS and Windows.