All of the game you have mentioned are excellent and all Sega Genesis fans should be playing them on a regular basis. For my Top 10, I decided to choose the videogames that I played and enjoyed when they were new. Thunder Force 3 was a sensational rush for me in the Winter of 1990, and that was at the time when Genesis was really coming into its own, before the arrival of the Super Nintendo and the real 16-bit war. I often preferred those early years the most. Same goes for Fire Shark and Gaiares, of which I was very proud to own and played endlessly (even if I needed to rely on the cheat codes for Garaires).
Thunder Force 4 was always a bit of a mixed bag for me, at least the US "Lightening Force" that was released in the States. It's a spectacular game and a Technosoft classic, but I was often frustrated by the slowdown that would frequently occur. The early stages also lack the immediacy and punch that TF3 and Garares provided. It really hits its stride at the mid-point, however, and once your ship receives its subwoofer upgrade, TF4 is on fire. I especially loved the volcanic stage, with rocky terrain in the foreground and the line-scrolling waves in the background. The final boss battle was another spectacular highlight.
I can certainly understand why TF4 remains so popular today; in fact, it was voted the most popular shoot-em-up on the Genesis once all the polling data was compiled, which was a nice surprise.
I ought to follow this by saying that Technosoft reissued TF4 on Sega Saturn on the Thunder Force Gold Pack series, and that's the definitive version to play. Why? Absolutely no slowdown whatsoever. I think Genesis fans who took advantage of the slower moments to dodge enemy bullets will find themselves overwhelmed here. Apart from overclocking your Genesis console (and why hasn't that been done on the emulators?) this is the best way to play.
I never liked Thunder Spirits on the Super NES, but to be honest, I hated the idea of my beloved Genesis masterpiece on Nintendo's system. Of course, I owned both systems and loved them equally, but at different times. When one system dragged, the other soared.