I´ve never used the Atlas on a regular base, mostly because it screams !Lyran! at me at high volumes and that is something that usually makes me shoot in that direction, but I got to say its reputation is not unfounded.
I never liked the weapon placement, though, and I have never grown fond (or rather not scared) of any upgraded variants with especially the 3050 variants giving me fits. The AS7-K2 and -K3 continue the terrible tradition of carrying huge honking explody weapons in ther side torsos and use XL engines. Yes, it is not as bad as many say, because you got to eat through that armour first before the explody bits are even a factor and I use Mechs with this flaw myself, but for me it goes completely against the character of the Atlas. It is meant to be the biggest, meanest kid on the block that just keeps coming at you regardless of what you throw at it. (and they look goofy, too.)
So far I have had no experience with or against it, but when it comes to gut feeling the AS8-D is what I am looking for when I want an Atlas. Finally TSM, so you can really throw other Mechs around like toys! That is what I expect from an Atlas! Hostile Wasps and Stingers are not a problem or even a nuisance, but useful close combat weapons! I also like the fact that your arm weapons won´t get used in spitting range anyways, so you are free to hug anyone that comes your way. Yes, your long range armament is lacking, but as others have said if your assault Mechs aren´t assaulting a location or defending it against an enemy assault, you are doing it wrong. And the AS8-D is built to be in the thick of it and still walk away.
Honorary mention goes to the AS7-Dr. Of course it is not on par with other modern assault Mechs, but if you consider that it is merely a refit (below depot level if I am not mistaken) which forbids many design changes considered essential nowadays (like DHS and CASE) it is quite sensible.