I feel motivated to get better heaters for my bettas, at moment i got the flat constant running heater, cheap ones., i just unplug when it gets to 80 and plug in when it gets to 75. good system if you remember. So anyway i am going with a more better heater. what would you get though for a 5 gallon

2-10 gallon heater
5-15 gallon heater
10-20 gallon heater

I want it to be energy effeciant as possible. i was thinking the 5-15 so it wouldnt struggle to keep the temp up all the time to 80-82