That's a tough one. For me, it comes down to basically a 3-way tie: Thors, Siege Tanks, and Ultralisks from Starcraft 2. When you see (a) Thor(s)/Ultralisk(s) marching into your base, it can get pretty intimidating, considering these are the largest units in the game and can take a shit-ton of punishment, while at the same time, dish out a beat-down of their own. Siege Tanks are more sentimental to me though, as nothing says hello like 90mm explosive shells raining down upon an advancing army or enemy base defenses from an incredibly large distance away.