You definitely need to play more than one game with each champ, you should at least be playing 10 per champ that you see as a possibility of you playing, and rejecting others that you think aren't you.
You need to play a lot of these champs because if you are going to main one, and you are using stats to decide whether you do or dont play a champion, your stats will be WAYYYYYYYY more accurate if you have larger amounts of data available.
Additionally, no one stat is the "most important": different roles that champions play with have more or less of a certain stat/s depending on what their role in the game. So when you compile your stats, have different classes of champions as your factors and your data as your variables. Eventually, after creating a data pool from which you gather your statistical mean from, you will see that in each class of champion there will be one out of the lot that you do better on in general.
So say you have a tank class and an assassain class; in your assassin games you will want more consistent damage throughout your games, which will be vastly different from your criterion for a tank which will want to take more damage rather than deal it.
Once you have an outlier for each class of champion, either use the winrates and preferences as your deciding factors as to if you play them or not, OR you could take them (the new pool of selected champions) into even more games to gather more data in order to see which one you really do like more/play better on!
That is of course, if you use data to select a champion, you could always just wing it and be a meta slave or just pick a champ thats fun and stick with it :P :)
Goodluck!