Developers with Genesis Allegiances!
It seems to me that a lot of the companies out there seemed to practice a bit of favouritism during the 16-bit era, with companies like Konami and especially Capcom and Squaresoft always giving priority to the SNES and Nintendo systems in general, so who were rooting in Sega's corner?
Off the top of my head -
Definately Treasure as they never seemed to make any games for other systems and openly trash talked the SNES :D
Virgin Interactive seemed to make their games on Mega Drive first and port later, also Dave Perry who was one of their major coders openly admitted to being a Sega fanboy on many occasions
Namco seemed to bring many of their highest profile games to the Mega Drive as exclusives (which was strange seeing as most of the Japanese devs seemed to be in Nintendo's corner, maybe this was due to Nintendo's censoring policies?) with the Rolling Thunder 2 port (and III being made specifically for the Mega Drive), Splatterhouse series, Marvel Land
EA seemed to put most of their effort into the Mega Drive with all the better versions of sports games as well as producing loads of European ports of Amiga software.
Technosoft - a bunch of high quality exclusives such as Thunder Force IV, Herzog Zwei, Elemental Master
On the Fence
I think Taito were pretty fair with their 16-bit output, it's just a shame they didn't bring some of their RPG's to the Genesis (though I think I heard somewhere that a Genesis version of Lufia was planned at one stage)