Gens32 is a Sega Mega Drive/Genesis emulator for Windows by the Gens32 team, which is based on Gens and adds lots of features.
There are 4 editions available at the moment, 1.x, 2.x, Surreal and Ray.
The most powerful edition is 1.x . The best versions in 1.x are Gens32 v1.78 Perpetual and Gens32 v1.79 Mirror. The v1.78 Perpetual supports both LDU and caption system (something like a DVD caption, but for games). They also have a better GIM (Gens32 Input movie) support. But, v1.78 & v1.79 are a little weak in 16bpp mode.
The Second edition is 2.x. The Best version is v2.35 Secret. It worked fine both under 16bpp & 32bpp. It's also powerful in terms of input.
The latest & active version is Surreal, which is powerful in sound. It also has a good compatibility, especially regarding hacked roms and Chinese roms.
Gens32 Ray is an episode during Surreal's update. Just use Surreal instead, it's better now.