If you are into B&W, and do your own processing, select a 400 speed, traditional B&W film (HP5, Tri-X), shoot at an EI of 200 and process for 20% less time than would be normal for that film/developer combo. This will give you a fair amount of latitude. The Canonnet uses the PX625 battery which is no longer available, and depending on what type of battery you have in there it may or may not be giving you spot-on exposures, so the latitude would be of help.
If shooting color, use a 100 speed, or 200 speed C-41 (negative film) and go for it - there is plenty of latitude for you there.....
If you want to shoot transparancy film, use the canon on manual mode, and use an external, hand held meter to determine exposure.
Enjoy!