4. Formalize your criteria. This is, in my opinion, the most important aspect of choosing a list topic. A good list has very formal criteria for inclusion and ranking. The benefit of formal criteria is that it makes your list less opinion-based: it gives you something standard and concrete against which you can weight the games' merits. If someone comes along and disagrees with your list, the disagreement takes place in the context of your criteria: do they disagree in how you applied your criteria, or in your criteria themselves?
Recent discussion on the board has mentioned formal criteria a lot. For example, one poster is proposing a "Top 10 Non-Main Characters" list; but how do we define main character? Is it the primary playable character, or all playable characters? In Monistic_Turtle's Licensed Song/Musician list, his formal criteria were that the music had to have come from an outside artist not working directly on the soundtrack. However, his criteria did not define whether public-domain music counted, nor did it explicitly state that music-based games weren't considered. The list was still excellent, but a lot of criticism centered around a lack of stated formal criteria.
So, state your criteria. For some of my lists, some examples of formal criteria include: the game was released at least a year after the movie (Top 10 Delayed-Release Movie-Based Games); companies that had an exclusive partnership with Nintendo, evaluated solely on the games developed during that exclusive partnership (Top 10 Nintendo-Owned Or -Affiliated Game Developers); and priority is given to artists that have lent their voices to a variety of roles, rather than one very good role (Top 10 Voiceover Artists In Gaming). The latter is an especially good example. Many people blasted that list because it didn't include David Hayter, but that was addressed in the formal criteria: Hayter did one very good role, but didn't have variety, and thus wasn't considered highly for that list.
5. Ask yourself: Is this interesting? Why? A problem that can come up in writing lists is that people get bogged down in the nitty gritty of just sufficiently describing everything to get the list posted. That's why many lists are boring: the writers themselves forget to make them interesting. If you don't enjoy writing the list, chances are no one is going to enjoy reading the list.
So when you come up with your topic, ask yourself the question: would I want to read a list on this topic? If you wouldn't, chances are no one else would either. But more importantly, once you decide an idea is interesting, ask yourself: why? Why would this idea make an interesting read? The reason to answer this question is because you want to write specifically to this motivation: if it's interesting because it highlights funny moments in games, then make the list funny. If it's interesting because it highlights a side of gaming people might not know, then make sure to include lots of trivia. If it's interesting because of an internal contrast between games, then accentuate that contrast. The important thing is to identify why a list topic would be interesting, and then write to explicitly exploit that reason.