Except most filters don't use all that much electricity so other devices may use more. For example: my AquaClear 70 uses 6W of power or 144 W/h per day. My lights on the same tank use 2x24W for 8 hours a day which equals 384 W/h per day. Depending on how often your heater turns on, it may use even more than that.
In my situation, it's the heaters. In the summer time, I use half of the electricity compared to the winter. In the winter time, heaters will be on most of the time and heaters run from 50w-300w.
The only correct answer is it depends on what equipment you're running.
If it's really cold where you're housing your tank, maybe it's your heaters. Other people live in tropical climates and barely use heaters.
If you have high wattage lights and tend to run them a lot (maybe a planted tank?) it might be your lights. But some people only run their lights for a few hours a day and/or use something like LEDs which use very few watts.
If you are running a huge pump 24/7, it might be your filter. But you could be running a HOB that barely uses any electricity.