EmuNations

SameBoy is a user friendly Game Boy and Game Boy Color emulator that's very accurate and includes a wide range of powerful debugging features. In addition to accuracy and developer capabilities, it has features such as Battery save support, states save states, scaling filters, multiple settings of high-pass audio filters, real time clock emulation and much more..!

• Supports Game Boy (DMG), Game Boy Color (CGB) and GBC-Mode Game Boy Advance (AGB) emulation

• Supports accurate high level emulation of Super Game Boy (SGB; NTSC and PAL) and Super Game Boy 2 (SGB2)

Gambatte is a portable Game Boy Color emulator. Its accuracy is among the highest and is based off numerous reverse engineering tests and documented studies. It is currently available as both a core for RetroArch, as well as an individual emulator. That said, it's not actively developed anymore.

As such, you can always check an active fork gambatte-speedrun which was also blessed by sinamas. 

mGBA is an open-source modern Game Boy Advance emulator under active development that also supports Game Boy and Game Boy Color games. As of yet, it's the most complete GBA emulation effort and is also available as a libretro core on RetroArch.

BGB is a Gameboy emulator which lets you run Gameboy and Gameboy Color games. It has excellent accuracy, gamepad support, high quality sound and graphics, smooth V-sync animation, and low latency. In addition, it contains a debugger that lets you analyze/look into the emulation, create cheat codes, and assist in creating and modifying roms.

Visual Boy Advance - Merge (VBA-M) is a fork of the inactive open-source emulator Visual Boy Advance. It mainly targets Game Boy, Game Boy Color and the Game Boy Advance though somewhat offers emulation for Super Game Boy and Super Game Boy 2. While supporting all previous features, it adds Link-cable emulation between GBA gamepacks, Joylink GCN/GBA emulation, e-Reader(still in development), etc. It has a very good accuracy level and is available for Windows, Linux, macOS, BeOS, GameCube and Wii. 

Gearboy is an open-source Game Boy/Game Boy Color emulator written in C++. It's not the most accurate, but runs on iOS, Raspberry Pi, Mac, Windows, Linux and RetroArch.

SkyEmu is a low level GameBoy, GameBoy Color, Game Boy Advance and DS emulator. Its primary focus is to provide a good user experience through a good mixture of tradeoffs of accuracy, performance, features and usability...

• Highly accurate Game Boy Advanced emulation

• Game Boy and Game Boy Color Emulation

• Experimental Nintendo DS support (only capable of running homebrew currently)

• Cross Platform: Windows, MacOS, Linux, Web App (with touch screen controls for iOS and Android)

• Game Controller and Rumble Support with configureable keybinds

• 4x Save Slots with screenshot preview

• Game fastforward and rewind support (supporting very long rewind times)

• Support for emulating the Real Time Clock

• CPU, MMIO, and Memory Debuggers

• Dark and Light Themes

• Support for loading official BIOSs dumps

No$GBA is currently among the fastest working GB emulators, though not the most accurate. It supports debugger with graphics user interface, conditional breakpoints, symbolic debug information, VRAM viewer, optional warning messages, online assembler, and various other useful things, making it a powerful development tool.

Ares was originally Near/Byuu's last project and was subsequently continued by new developers (including Luke from Cxbx-r). That said, it's an open-source, multi-system emulator focusing mainly on accuracy and some of its exhaustive features include but are not limited to native multi-platform UI adaptive sync, dynamic rate control, save states, run-ahead, rewind or fast-forward and much more..!