I've never hired a car in Tas (I live here
). Avis, Hertz, Europcar ... I'd look at the majors like these, then go on price (assuming all are unlimited kms). Smaller or distinctly cheaper operators may not have a presence at the actual airport, meaning you have to be picked up and taken to a lot off the airport precinct. Not so much a big deal on pick-up, but inconvenient and takes time when you drop-off and have a plane to catch.
Be aware that car fleets have been significantly sold off this year, so supply is less than usual. Don't wait too long to book..
If you have never hired a car before:
* You may not get the car class you booked - if you book a medium car and get a large(r) one, that may suit but parking trickier and more fuel. Shouldn't be charged more if they give you a bigger car.
* Look very carefully at the condition of the car before signing for it - dents, scratches, windscreen chips. Note them on the paper provided and insist that any are recorded and *signed* by the car agent. Sometimes these days its a bit 'remote control' and you may have to find the guy to get the acknowledgement. Check for the spare and that all the tools are in the boot.
*
Check to see if you are insured if you go off bitumen roads. There are a lot of scenic places in Tas where you go onto the dirt
* Understand the insurance options and choose an excess that works for you. Be aware that if there is an accident involving damage, they will keep your excess even if its demonstrably not your fault, until their insurance sorts it out.
Lastly, check driving times. Tas highways tend to be winding with fewer overtaking areas than you may be used to, so journey times are longer than may look on maps.