Rank the best Open Source videogames. It doesn't have to be Free Software, just Open Source. It should be games that the developers voluntarily made the source code available and allowed modifications.
Ranking the best open-source video games can be subjective, as different people have different preferences. However, here's a list of some popular open-source video games, in no particular order, that have gained recognition and appreciation from the gaming community:

1. [Battle for Wesnoth](https://www.wesnoth.org/) (Turn-based strategy)
2. [0 A.D.](https://play0ad.com/) (Real-time strategy)
3. [SuperTuxKart](https://supertuxkart.net/) (Racing)
4. [OpenRA ](https://www.openra.net/)(Real-time strategy) 
5. [FreeCiv ](http://www.freeciv.org/)(Turn-based strategy) 
6. [Xonotic](https://xonotic.org/) (First-person shooter)
7. [OpenTTD](https://www.openttd.org/) (Business simulation)
8. [Warzone 2100](https://wz2100.net/) (Real-time strategy)
9. [The Dark Mod](http://www.thedarkmod.com/main/) (Stealth)
10. [Cataclysm: Dark Days Ahead](https://cataclysmdda.org/) (Roguelike)
11. [Red Eclipse](https://www.redeclipse.net/) (First-person shooter) 
12. [Cube 2: Sauerbraten](http://sauerbraten.org/) (First-person shooter)

These games cover various genres, and their source code is available for anyone to modify or contribute to. Keep in mind that this list is not exhaustive, and there are many other open-source games out there.
Okay, but is it possible to mod those open-source video games?
Yes, it is possible to mod open-source video games. Since the source code is available, you can modify the game's code, assets, or mechanics to create new content, features, or improvements. The open-source nature of these games encourages community involvement and collaboration, allowing for the development of mods, expansions, and even entirely new games based on the original code.

For example, Battle for Wesnoth has a large community that creates custom campaigns, units, and maps. Similarly, OpenRA has mods that bring new factions, units, and gameplay mechanics to the game.

To mod an open-source game, you'll need some programming knowledge and familiarity with the game's codebase, as well as any required tools or software for editing assets like graphics, sounds, or maps. The specific process for modding each game may vary, so it's essential to consult the game's documentation or community resources for guidance.