I have often had occasion to blog about the Tandy 1000, but in terms of gaming, its importance is rather limited. I am a proponent of sticking to the original development system for a game. For example, practically every game published by Richard Garriott (before and with Origin) in the 1980s was developed on the Apple II. Most of the 1980s U.S. computer game developers cut their teeth one way or another on Cupertino's machine. His games really do not have a whole lot to offer on other systems. They don't take much advantage of the enhanced graphics and sound of the Atari 8-bit, ST, Commodore 64 or Amigas. Jordan Mechner's Karateka and Prince of Persia may look more colorful on other systems but play the same. For later games, FTL released their seminal dungeon-crawler classic Dungeon Master for the Atari ST and the game looks the same on the PC, even though it supports VGA. Sid Meier's Pirates! is a classic game that plays very well on its original platform, the Commodore 64. Not all games fall squarely into this category. Will Wright's SimCity began life on the Commodore 64, but by the time it was ported to 16-bit machines, it had gained many new features and functionality. Even so, it received Amiga and Macintosh releases before it was released on the PC.
Typically, the main advantage of the PC over every other platform is the prolific availability of hard drives, even (relatively) for the earliest machines, many speeds to choose from and a huge variety of clones to choose from. Who wants to put up with long load times, floppy swaps and inflexible system speeds? The earliest PC games would have been developed for the IBM PC, but most of those games were ports or otherwise historically unimportant and not usually rigidly wedded to IBM's hardware or the CPU speed.
The first historically significant PC game that was released and was not a port was King's Quest. This of course was originally released for the IBM PCjr. The PC version was released soon after and Tandy 1000 version would have been in existence by 1985. The Tandy 1000 version is nearly identical with the PCjr. version. King's Quest II came in one version with support for all three architectures, PC, PCjr. and Tandy 1000. However, it is clear that of these three systems, the best one to play any of Sierra's AGI games is the Tandy 1000. Not only do you get the enhanced graphics and sound, you do not incur the performance penalty of running the first 128K of the PCjr. on the Tandy 1000.
Thus is my definition of a "Tandy Original". First, it must not have been originally developed or released for a non-PC system. No ports from an Apple machine or Atari or Commodore non-IBM PC compatible. Second, the game supports the Tandy Graphics Adapter but no EGA or VGA support, or in the alternative the game supports the Tandy Sound Chip but has no Adlib, Sound Blaster, Roland MT-32 or other sound device support beyond the PC Speaker.
Thus for the AGI-engine games after King's Quest, they were probably developed mostly on Tandy and PC systems. Even King's Quest probably had more work done on it on the PC than on the PCjr. due to the not-insignificant advantages for the PC. When the hard drive installable versions of the games were released, starting with King's Quest III and Space Quest, these were easy to do on the Tandy 1000 and PC compatibles. Since hard drives were an expensive third-party add-on for the PCjr., fewer of these systems enjoyed this feature. I classify all the AGI engine-games as Tandy 1000 originals with two exceptions : King's Quest IV, which was developed for the SCI system and supported much more advanced sound hardware than the Tandy chip and Donald Duck's Playground, which was originally a non-AGI Commodore 64 game.
None of the other games Sierra released during the mid-to-late 80s qualify as a Tandy original. The Disney games were Apple II originals, the Game Arts games were originally developed for the NEC PC-8801. 3-D Helicopter Simulator does not take advantage of Tandy sound and supports high-res EGA. Thus 11 games from Sierra can be called Tandy 1000 originals with one PCjr. original.
While LucasArts, then Lucasfilm, did not have true Tandy originals, in Maniac Mansion and Zak McKracken their Enhanced versions qualify as Tandy originals. The low-resolution games were developed for the Commodore 64, but the high resolution enhanced versions were clearly developed on the PC. The Amiga and Atari ST versions do not look or sound any better than the PC versions. These games do support EGA graphics. When run in a PCjr., these games will display CGA-quality graphics and PC Speaker sound. The low resolution versions behave identically.
Similarly, the first two games in the SSI Gold Box series, Pool of Radiance and Curse of the Azure Bonds, also had support for the Tandy sound chip and nothing more advanced. However, these were ports from the Commodore 64. On a PCjr., they will need the Tandy mod to show the graphics properly. Hillsfar does not use Tandy Sound and Champions of Krynn, Secret of the Silver Blades and later games support Adlib.