The 400/800 has that cool futuristic 70s look but IMO the 800 is somewhat huge so the 400 has it's place as less of a space hog.
The 600XL comes into it's own in the modern day thanks to smaller components that allow internal upgrades such that you can equal any other machine except maybe for an internal mechanical HDD. 600XL is also a more sensible shape than any other 8-bit Atari computer IMO.
Later on the XEGS and 65XE became the povvo models. 65XE sort of an orphan especially if it's the NTSC one without ECI port, given that the form factor is the same as the 130XE.
XEGS - they screwed that up royally with the ridiculous short keyboard cable plus the fact the console keys are on the computer, and deletion of ECI. But there have been some well modded GS machines.
But overall, for the low end machines I'd go 600XL every time.