1-K and the 2-M are the best variants of the design. The 1-K is one of the best 3025 heavy designs, albeit somewhat underestimated because of the current love of zombie flashbulbs. It puts up a substantial amount of damage through bracket firing, and it can stay in the game for a very long time. The only thing it really has to worry about is TAC's, because a quick shot to the engine to a mech with 10 single heat sinks more or less junks the mech and forces its withdrawal. I've found the risk of ammo explosion is overblown: 14 tons of armor is as much as a Battlemaster, and if you have more than three of your ammo bins loaded when your armor is breached, then you've done something wrong.
The 2-M keeps one of the best assets of the 1-K (combat effectiveness in a cost effective package), while making the design somewhat more durable and better at fighting at all ranges. With the inclusion of an Artemis and Gauss, it has increased firepower at extended ranges, and because of the inclusion of double heat sinks, it can effectively alpha rather than bracket fire. Additionally, the 2-M is in my experience better at long campaigns than the 1-K because it is somewhat less reliant on ammunition based weaponry. The only real drawback the 2-M has over the 1-K is that it is less effective as a stand alone design: it has marginal critseeking ability, and without the SRM it is somewhat less effective (and wasteful) to use it against vehicles.