Good movies to compare are those with more than one of the following in common with the movie you're trying to predict:
- target audience (eg young males, adult African-Americans)
- franchise (prequels, sequels, reboots, etc)
- genre (eg adult drama, slasher horror, teen comedy)
- creative talent (eg actors, directors)
- release date (same month, same weekend, etc)
- release patterns (eg mid-week releases, platform releases)
And sometimes there are no good comparisons. When there are none, it's better to recognise this and maintain an open mind than shoe-horn in an unsatisfactory comparison - even if it is the best one available.
If you're looking for comparison ideas, you'll often have movie prediction sites - particularly boxoffice.com and Comingsoon.net - make comparisons between upcoming movies and past releases in the week of release. However, the most comprehensive source for this type of information is easily Box Office Mojo's individual movie pages, which all have a tab for "Similar Movies".
It is also a good idea to discuss what movies would make good comparisons on the Movies Board. You'll find lots of strong opinions, and debating what movies would (or would not) make good comparisons is part of what the discussion boards are for.