For a true icon, it would have to be the Atlas or the Timber Wolf, for me, though the Urbanmech is the meme icon (memicon?).
While the Warhammer graced the cover of the boxes for a decade (and its predecessor and look-alike Hammerhands a little later), it runs in to the Unseen problem head-on.
The Atlas is unique, part of the original TRO: 3025, and the only thing it doesn't bring is being part of Battledroids (but those all have the Unseen issue).
With the introduction of the Clans, we have the Timber Wolf. It graced the cover of the Compendiums. It was the most commonly seen Clan Mech of The Battletech cartoon (even if we never saw a single pilot of one). It's also a great blend of speed, armor and firepower along with its unique and frequent appearance. They've made 3 more chassis based off it now, too!