What I do is book online with one of the major companies, usually Europcar, and pick the smallest, cheapest car on offer. When picking the car up I'm polite, attentive and listen carefully to the whole spiel, before agreeing to buy the extra insurance. Without fail this gets me an upgraded car.
This summer at Porto airport the Renault Twingo which I'd bought and paid for became a Clio. The bill for the extras (insurance, toll tag and all the tolls) came to less than the difference in base price between the two cars.
Best ever was Arlanda airport at Stockholm. Only the top rate of insurance covered for stone chips, which were quite likely as our accommodation was 15 miles from the nearest bit of tarmac. By agreeing to each level of insurance in turn with a bit of mild haggling thrown in the VW Polo I'd booked gradually morphed over a series of upgrades into an Audi A4 Quattro TFSI.

Great fun on gravel roads.
But if you only want a Twingo or a Polo ......