Stage 3, R-Type
In the R-Type scrolling shooters, the boss of Stage 3 is Stage 3 - a massive war machine that you spend the entire level picking apart, gun emplacement by burner rocket, while avoiding being shot down or crushed beneath its moving parts.
Above: That's not a roof above you, but about a fifth of R-Type Final's destroyer
The original R-Type's battle cruiser gets the nostalgia vote but the PlayStation-only sequels have the most imaginative reruns. R-Type Delta on PSone offers a stomping great four-legged walker, while PS2's R-Type Final has a mothership so massive that it features a parking garage big enough for a giant robot sub-boss to pop out of mid-way through.