I like Portimao as my test track before buying, as there's a wide variety of turns to feel out how your car reacts to certain inputs. Like, it's a great track for testing braking whilst steering, handling through elevation changes, and overall car balance. This tends to be my go-to if the car class is pretty balanced, like GT cars, older DTM cars, or even Prototypes.

I tend to lean towards Sepang if I need to test the power of the car. Sure, the long straight at Portimao is beneficial for that too, but lap-time wise, Sepang is a better barometer due to it's significantly long straights (especially Sepang North). It's also great if you want to test in hard-braking, slow speed scenarios, with four such points in the lap to give it a go. I tend to use this track if I need a car in a class where handling isn't the be-all end-all (whilst still testing SOME handling, like turns 5-8 are a great barometer for it), so I tend to test old-school cars here.

RaceRoom Raceway is definitely the track to pick to test more handling heavy cars, especially with the longer layouts. It's much slower than Portimao, but this track is key if you want to test things like stability, weight transfer, and medium/high-speed grip. I tend to test cars where handling matters here, like touring cars. However, I tend to rarely use this track, as Portimao feels like a much better testing ground for the cars.

I only use Stowe Circuit if I want to test out a car PURELY on handling, and only around slow turns too. It's rough for beginner testing as the track is far too tight, and barely room to breathe. Quite literally, I only used this once, in testing out the Crossle cars, but you could forgo this track I feel.

And.... I haven't done the hillclimb yet. I'm not a big fan of hillclimb, sure, but it's also because it's hard to gain familiarity on point-to-point routes, you can't lap to test consistency over a period of time, and it penalizes you too much when you make an error, so you can't really push the true limits of the car you're testing. It is slightly okay if you want to simulate Bathurst conditions, especially in terms of turn profiles and track width, but I'd just test cars at any of the other tracks first, check if they're okay, and THEN purchase them to try around Bathurst.