JTippetts has a compelling argument for picking something you like, but sadly many beginners don't start small.
I note that JTippetts' goblin project looks like it is now in year 11. That many years, even at a hobby level, disqualifies it from being a beginner project. Over the course of a decade you can move from beginner to veteran.
Most beginners aren't looking for a decade-long project.
"I want to make an MMORPG" is the common refrain in the For Beginners forum. After a brief explanation about what "MMO" means, the next week it becomes "I want to make an Online RPG", then after they post a few times in the online forum and discover it requires work, a few weeks later it becomes "I want to make an offline RPG", then if they bother to follow up, becomes "game programming is stupid. What are good free game makers?" And then a few months after they appeared on the board, they vanish after learning that while playing games is fun and entertaining and a diversion, it is different than making games which requires thought and effort.
My recommendation for beginners is the same as my recommendation for everything: Build the simplest thing that will possibly work. The philosophy has worked well for me for two decades.
Pong is a frequently suggested game because it seems relatively simple. The rules are simple. The design can be expressed in one written page. But for a beginner implementing such a system, it can feel like barrier after barrier. You have game loops that are beyond their experience. You have graphics systems that are beyond their experience. You have input systems that are beyond their experience. You have audio systems that are beyond their experience. You have collision detection and collision response that are beyond their experience. You have game play logic that is beyond their experience. Every new system has a learning curve.
When someone says "I want to make an RPG" that can be good. If they mean to build a tile-based world with animated sprites on the grid with a simple menu system and a simple menu-based action system for everything, that isn't too bad and with the right tools can be put together quickly in the matter of a few weeks. If they want to build a massive 3D world with articulated 3D characters holding 3D weapons each with their own animations, thousands of monsters each with their own animations, with hundreds of thousands of unique dialogue lines and dynamic story lines .... well, that is still an RPG but is something altogether beyond the skills of a beginner.