While it is primarily an auction game, Ra also incorporates push your luck and set collection as mechanics. Knizia blended those three elements together into a tightly knit whole, with each element supporting the others.
Here's the game in a nutshell: players take turns either pulling tiles out of a bag or calling an auction for the tiles that have been pulled out. The kicker is the Ra tiles. Drawing enough of them will and the round with no one getting any more tiles.
The variety of tiles in Ra is greater than I can easily summarize. And they all score or lost points in different ways. It's easily the most complex part of the game and the hardest part to teach. Of note are the disaster tiles, which will actually make you lose other tiles.
And the last element I want to make sure I mention as the sun tokens, which are what you use for bidding. They limit how much you can bid and you can only use one at a time. This helps prevent people wildly overbidding and also limits how many auctions you can win on each round.
Oh, since I haven't actually bothered mentioning the theme before now, I figure that I should cover it. The game is set in ancient Egypt and each round is supposed to be an epoch. It actually works rather well, since you are developing technologies and fostering Pharaohs and building monuments and things like that. I mean, the game has been re-themed for gangsters but I feel that the theme of the tiles does marry well with the idea of the passing of ages. Plus, cool Egyptian art.
Now that I've given a very brief thumbnail sketch of Ra, leaving out details like automatic auctions when you pull a Ra time, why do I think it is such an awesome game?i
The push-your-luck element is strong enough that Knizia used similar mechanics as entire games, like Cheeky Monkey. It adds a lot of fun and tension to the game. Every that only one person has bidding tokens and is pulling tiles to try and get s great haul, it seems that everyone else starts chanting Ra, Ra, Ra.
But that push your luck element is countered by the diversity and complexity of the tiles. Different tiles will be worth different amounts of points to different players. Understanding both what you will get out of a given lot of tiles and what other people will get is very important.
There's a strong tactical level to Ra, about dealing with what tiles come out. If certain tiles don't come out, you can't plan your game around them. But the game is also very strategic as well. You need to have a good sense of how the tiles will score as well as what's still in the bag. In the long run, a global understanding of the game situation will trump any given lot.
Ultimately, that's what makes Ra such a strong game. The push-your-luck element creates a lot of constant tension and makes it possible to have a lucky break or an unlucky fall. But the complexity of the tiles makes having a global understanding of the game very strong. The better player should win Ra but they have to work at it.
The diversity of the tiles and the luck of the draw also gives Ra a lot of replay value. You can't play the game on autopilot or try and follow the same formula every time. You have to react to what Ra gives you and build your plans around that.
Ra came out in 1999 and, while it has gone out of print, it seems to reliably always come back into print. The core mechanics of the game are simple to explain but the game has some serious depth, as well as a truckload of fun and replay value. I consider it to be a definite classic.