What topics are necessary to log for creating a SLAM comparison bag file?

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For gmapping or karto, you need transform data and laser scan data. So you should record /tf and the topic with your laser data, which is often /scan. Note that the laser scans should be from a fixed, horizontally mounted laser. On some robots (e.g., the PR2), this data is on the topic /base_scan.

It's easier if you don't run a SLAM or localization system (or anything else that estimates the map->odom transform) while you record the data. Otherwise you'll have to filter out that data from the bag (how to do that is another question).