Back seat with harnesses on.
I have an estate, but with all the dog walking associated stuff that's in there, I'm not sure I'd fit them in, as I'd need at least 2 crates, as Ms Daisydoo would pick on the boys if crated together! Also, alot more window there if travelling on a sunny day (back window & 2 small side windows) & nowhere for them to escape the sun - on back seat, one in the middle's ok anyway, & the ones on either side have worked out as long as they don't move suddenly, the seat belt extends enough so they can get in the footwell at the back out of the direct sunlight .... I had those mesh things that attach to the window with a sucker, but they couldn't nosey out the windows properly, so kept pulling them off!
Just a thought for anyone travelling with their dogs on the front passenger seat, my daughter used to do this when she just had one dog, until it was pointed out to her that she would need to disable the passenger airbag, otherwise, if they were in an accident & it was activated, it could suffocate her dog!