Should start with the fact that this may come down to personal taste and mileage is going to very allot.
That said, my 2 Cents; While there are some amazing Fasa era covers, some have aged terribly.
Honestly, as much as I love the mechs of the setting, many of the most boring covers are just mechs are the most boring. Some give as a great dramatic angle that gives us a sense of movent and a upward angle to give a sense of scale (we need to look up to see it) but too many are just robots shooting robots. Also not the biggest fan of the colors ROC chose to use for their covers in the early-mid 90's, giving Battlemechs the same colors as Transformer and GoBots. It was less space opera and more Saturday morning cartoon. A good number of covers would improved 100% with better use of colors.
DA novels are a mix bag but someone a while back pointed out most of the covers always put the characters up front much like the early FASA novel covers. Those 90's ROC covers I had a problem of looking like they were trying to sell you a toy, the ROC DA covers look like a DVD cover... though some of the DVDs you would find on the bargain rack of a gas station.
The current CGL covers have a good mix, the mechs are front and center again but rarely boring pose (a few maybe but that might just be me) and the character's profile in the background reminds you this is a human story, not a toy ad. It reminds me of a few source book covers like Shattered Sphere and FedCom Civil War, the action is on the for front but this looming figure is the driving force of the conflict.
... and no, I'm not going to get hung up on what mech is on the cover. If I was, ROC would owe me dental money for grinding my teeth from 1995 on from this gem:
I remember my friends and I spent too much time trying to figure out which mech that was suppose to be. My 12 yo brain couldn't accept a professional artist only used one point of reference for the Dire Wolf. Roger Loveless has a impressive portfolio but you can tell ROC gave him a deadline and he wanted to cash the commission check. So no, not going to get hung up on what Warhammer was use on the cover, as long as I can tell it's a Warhammer.