There is no such thing as "ultimate system" for me. I like many different play styles and every game that tries to fit them all works poorly for most. I need multiple games, each designed for specific style and setting.
Anyway, there are some common traits that each game should have, IMO:
0. Be clear and honest on what it is about, what it does and how it does it. I love games with a lot of sidebars explaining designer intent behind various rules and setting elements.
1. Rules that focus on what is important for the game and ignore what is not. If the game is about adventuring, don't waste my time and money on crafting rules. If it's not about combat, don't put a chapter about fighting in there.
2. Rules that are easy to use. They may be more or less abstract, more or less complicated, but after a single session of learning them they should be intuitive enough to use without much looking up things in the book during play.
3. Single book that is all I need to play. If I need two books of rules, a separate book with monsters and one more with setting description, the game has lost me already. I'll use my money and shelf space for four one-book games instead.
4. Rewarding a play style that fits the game. A good game focuses its rules on players that want to play it as designed (and communicates clearly how it is designed to be played). It does not spend a lot of rules on preventing people that want to abuse it from abusing it, because they will always find a way to do so.
A good game does not create a tension between playing a fun (interesting, genre-appropriate, multidimensional) character and playing an effective one. A good game is designed in such a way that interesting, genre-appropriate characters are the optimal ones.
5. Inspiring setting. It does not have to (and even should not) be described in detail. What I need is enough info to get the feeling and enough hooks to hang my games on. A few evocative sentences work much better than a few paragraphs of cold data.
Not all games I play fit the criteria above. Also, not all games that fit them are fun for me - there are a few games that I perceive as well designed, but have no interest in playing.
A few examples of games that are great for me - perfect in design, fun in play: Nobilis, Dogs in the Vineyard, Mouse Guard, Wolsung. Each one is different and each one is really good in what it does.
