Hard to say for sure, as it can be track dependent or grip dependent, and class dependent
Xray:
2.6, 2.7, 2.8, and 2.9 would be good to have on hand. If you want the progressive ones, the 2.3-2.6 and 2.5-2.8 are what xray has. I'm mostly around the 2.6 all around or 2.6 front and 2.7 rear at the moment, or that's what's currently on the cars anyway.

SMJ:
They are a bit cheaper to get and do now have some rates that are a lot closer to what xray has.
Red progressive front and blue rear was what I had ran last on those, and they were ok. I have ran the red progressive front and yellow progressive rear before and they were good in USGT, was going to try the yellow front and red rear setup, but didn't get around to it.

Axon:
i haven't ran this brand, but have looked at what they have, and they do seem to have a much finer set of spring rates from when compared to the other two.
for example they have 2.5, 2.6, 2.7, and such like Xray and now SMJ do, but Axon also has 2.55, 2.65, 2.75, and such that fall between the rates. Don't know if they're good or not, but I could see the finer rates being worth trying.

https://ultitires.com/

this is where I get my springs from. Get the package springs rather than buy individually. I have bought duplicates of a couple just to run the same front and rear.

Moderators......wasnt sure if I could post the link I did. Just trying to answer a fellow racers question. If I’m punished for doing so I can accept it.

the blue springs are linear. The progressive spring will have two numbers associated with their ratings. 2.5-2.8 for example.

i know there's a few that fit. Axon TC spurs is wat I use and I think what a lot of racers use at the local on-road track.

There's a number of places on line that carries them. Amain and Ulti Tires are the two I have got all mine from

what calass your running? Mod TC, the spur gear seems to be 110-114 I think, 17.5, likely 102-108 maybe, and 21.5/USGT 102-104. At least those are the general ranges I have or have run. All in 64p.