Hi JC,
This is such a subjective question. It depends completely on what makes you smile.
I am MECH COLLECTOR, with the goal of owning every single Inner Sphere chassis, every variant, and every Hero before I pilot a single Clan mech. So for me, it would be the Stalker.
But.. if you already own 4 of the better ones, I see little reason to buy a Stalker that can only AMS better than the rest.
The XL 255 is a 'must have' engine along with the XL300, XL280, XL225 and perhaps a few others.
Enjoi!
Edited by Flak Kannon, 09 June 2015 - 02:49 PM.