Sonic Adventure DX 2011 was already mentioned, it gets my vote to balance out GTAIV. I also want to sum it up/add my two cents.
-runs very badly (relatively)
-locked internal rendering resolution (vga my guess)
-omitted visual assets
-omitted content (game gear mini games)
-analog sticks are 8-way, only 4 of which are really analog
-configuration tool bug, users had to manually edit registry or move the settings file into a different folder
These may not be enough to make it the worst port ever, BUT:
-a perfectly good port already existed, there was no reason to produce this pc port of a x360 port of a gamecube port of a dreamcast game
It is playable, but so consistently bad that it needn't/shouldn't exist alongside the 2004 port.
Honorable mentions:
Sonic Generations with the Optimus incompatibility which gradually grew WORSE with every "patch".
Sonic Heroes with its broken special stages that are needed to complete the game. The later ones were simply impossible to beat without special knowledge of a speed glitch which you have to get around by COMPLETELY altering your playstyle. May sound like a minor gripe but it's not.