What bad game had the most unexplored potential?

Motorama. If it had good driving physics and better music - I would love it,

Outpost

Damnation

Firefall, the potential around it in closed beta was insane. Then they just **** it up. Oh well.

Battleborn

It's not a bad game but before Mafia II came out they promised a lot of features that never made it into the game. Like organically finding side missions for example through hearing a rumour while you're getting your shoes shined or overhearing a conversation. If I remember correctly there's ~ 5 GB of unused data in the game files.

The Division, Watch_Dogs, RainbowSix Siege, the first Witcher game, Halo 5, Deus Ex Invisible War, Mass Effect series, The Wolf Among Us,...

Yeah, I consider all of those to be bad games. At least The Witcher's story was pretty good, but the others have got no mayor redeeming factor for me that outbalances how they failed in delivering.

While it isn't the worst game I've played by a long shot, I was sad at the missed potential in Postmortem: one must die. It had a lot of cool ideas going for it, but the lack of individualizing the main choice you have to make really ruins it.

spore or the latest sim city come to mind

Raven's Cry
Jagged Alliance: Flashback
Call of Juarez: The Cartel
Enter the Matrix

Fighting Force
Resident Evil Survivor

Police: Enforcer Crime Action

Command & Conquer 4.
Story was badly written and does not fit in with the rest of the lore so far. (Remember how C&C 3 had a sequel hook with the Scrin alien invaders announcing a full-scale invasion to go after Kane ? Now guess which faction does not appear in the game...)

Then there's the weird-ass levelling system which just feels out of place, especially in the single-player campaign. It would feel better if the campaign was built around a more open-ended map, liked Emperor: Battle for Dune did.

Finally, they replaced the tried-and-true traditional base building with the roaming crawlers, making it a bit more like Homeworld on a 2D plane.Which is kind of a daft thing to do to cap off a subseries of a franchise to start with - better to spin that off into a subseries of it's own.
But the way it was done was also horribly undercooked, and just not fun.
If you're looking for the gameplay style done right, it seems that Homeworld: Deserts of Kharak shows how it should be done properly.

To the Moon. To quote an old post of mine:

...a rather average game which can be made to be an amazing one. At its current state I give it 6.5/10, but I think it has great potential. These are the changes I'd like to see in a remake with a larger budget:

Extreme changes:

• Switch to a modern graphic engine, with 3rd person perspective, highly detailed maps, characters and objects and proper human mechanics/motion. The current pixelated 640x480 view is sort of a turn off, and is probably one of the main reasons why people don’t finish the game (or why many of those who got it in a bundle didn’t even bother to play it). Think how nice indie games like Contrast or Brothers – A Tale of Two Sons look. Now imagine the story of To the Moon presented in such a way. Wow.
• Replace the dialog bubbles with actual voice acting, and add cut-scenes with two or more camera angles and, where needed, close ups on characters. Tomb Raider series uses this effectively. There are many others, though probably nothing with a budget similar to that of To the Moon. The Cat Lady probably didn't have budget much larger than To the Moon, but has amazing voice acting.

Tweaks:

• Make physical memory links and mementos easier to find (e.g. make them glimmer). Scanning a scene with the mouse trying to find memory links doesn't provide very good gameplay. It feels like work, especially in the larger outdoors maps.
• Get rid of the tile-flipping puzzles. These don't make much sense and aren’t any fun. A better alternative would be to present close ups of memory links (including their name) and have some animation that shows the machine automatically linking them (or find an interesting and sensible way for the player to link them).
• Unify the Memento Configure and Activate dialogs. I can’t think anyone ever chooses to cancel these dialogs, so why repeat asking?
• Have a different color for dialog bubbles (per character). This would make it easier and quicker to follow the dialog. Another option is to move the name of the character inline the text, like in a play or a script.
• Display the full content of Notes in the main game screen (as they are collected). What’s the point of having to switch out to the Notes screen to read them, when usually they are just one-liners?

Watch_Dogs

kingdoms of amalur.
tried to make a mmo, then work as a singleplayer game. BORING.

Any Battlefield game
Any Call of Duty game

Duke Nukem Forever

Check the DLC, The Doctor Who Cloned Me. It's actually good. Gearbox showed us what DNF would be if they made it from the beginning. Of course, it has the same mechanics(not so good, but they couldn't change everything in DLC - and they've finished the main game, not made it entirely, that's why it wasn't great if I remember correctly), but new stuff they added and new level designs - I enjoyed it.

"unexplored" as "underrated" or as "they tried to do a good thing but ended being something pretty much normal"?

Derrick the Deathfin... I followed this game when it was on the PSP and I voted for it when it hit greenlight. I watched it get greenlit and waited... and waited... then the game hit Steam and it was just kind of meh. I was expecting a more epic-feeling game (like Spelunky or even Adventures of Shuggy) and I got what amounted to a generic mobile-esque game.