My first picks:
-Carrier - Hidden Gem. A total rip-off of the old-school RE games. But a good one. Ironically enough, the ripping-off came full-circle somewhat recently with the release of Resident Evil: Revelations, which takes place on a huge cargo ship, just like this one.
-Dynamite Cop - Short, but fun beat-em up game with similar gameplay to Fighting Force, intermixed with high-octane cutscenes with quick-time events.
-House of the Dead 2 - Horror Light-Gun game that's perfect for fans of cheesy horror movies.
-Marvel vs. Capcom 1 & 2 - Fan-freaking-tastic 2D fighter by Capcom combining SF and other Capcom characters with Marvel Comics characters.
-Maken X - I almost forgot this one...shame on me. I had to come back and add it to my post. Fantastic, and unique action game. You can take control of different characters through the game by "mindjacking them", creating a different gameplay experience for each character. But at it's core, it's a 3D beat-em up/hack-n-slash.
-Rayman 2 - Best platformer on the system, IMO (eat that Sonic Adventure). Available on other systems, but this is the best version.
-Record of Lodoss War - Diablo-style RPG based on the fantastic anime of the same name.
-Sega Smash Pack - The old school Genesis games are great, but you can get better collections on other systems. The real draw here is the port of Virtua Cop 2, which is compatible with Dreamcast light-guns. If you don't have a Saturn, get it for this.
-Spider-Man - (Not the same game as the PS2/XBox/GameCube game based on the movie). Best version of one of the best action games and super-hero games of that generation. It is also available on N64 and PS1, but this is the best version.
-Vigilante 8: 2nd Offense - You like Twisted Metal? Then you will ADORE this game. More weapons, more vehicles, more areas to fight in; similar gameplay. And better graphics, to boot, since it's on Dreamcast.
-Zombie Revenge - Set in the same world as House of the Dead, but this time a beat-em up with similar gameplay to Fighting Force, but fighting zombies and monsters instead.
Honorable Mentions:
-Crazy Taxi 1 & 2 - A taxi game, no really. You race customers around for cab-fare (points) as fast as you can, running rampant over an entire city. A lot of fun, in short bursts.
-Quake 3: Arena - Great multiplayer FPS. I say honorable mention because it's really only good for local multiplayer. Since it's 2016 and all, and the PC version is online and the Dreamcast one isn't anymore.
-Resident Evil 2 & 3 - Best versions of these survival-horror masterpieces, besides the GameCube versions (or so I'm told, I haven't actually played the GC versions). Also available on PS1, and RE2 on N64.
-Resident Evil: Code Veronica - Fantastic old-school RE game. Honorable mention because it's available for later systems.
-Tony Hawk Pro Skater 1 & 2 - A time-killer game that's just a ton of fun. But like RE: Code Veronica, it's available on other systems.
-Unreal Tournament - Great multiplayer FPS. I say honorable mention because like Quake III, it's really only good for local multiplayer. The PC version is online and the Dreamcast one isn't anymore.