Thunder Force IV is the reason for a lot of the love the Thunder Force series gets, and for good reason, it was a technical tour-de-force on the Genesis with some of the best visuals of the generation and one of the finest OSTs on the platform, if not the finest.
Unfortunately, if you play TFIV today, you'll quickly realize it's a very memory-heavy shooter with a lot of stage hazards and unpredictable GOTCHA moments.
Enter TFV. It was the first 3D outing for the series, and consequently it's not aged as well in the visuals department, and while the OST is fun and one of the best in the series, it doesn't match TFIV's. It also saw a very limited US release (it was originally a Sega Saturn game, but only came to the US years later as a slightly inferior PSX port with a low publishing run).
But it's a LOT more fun to play than the previous games. The stages are well-paced, the weapons and bosses are cool, and the scoring system has been expanded from TFIV's mega-simplistic offering into something that drastically improves the game and encourages replays and score runs.
This is my favorite Thunder Force game, and I really feel it's been unfairly slept-on.