Halo: Combat Evolved
There is a point, a little over halfway through the game, where it - all of a sudden - becomes difficult. One or two levels seem to stretch on forever with near-infinite repetition of identical rooms and corridors. And the characterization is pretty weak (yeah, it gets worse). But this isn't enough to detract from the game's simple pleasures of anti-alien violence, bolstered by a mostly-great difficulty curve, addictive action-driven plot, and mood-setting music.
Halo succeeds in being fun in spite of its flaws; the game is greater than the sum of its parts. I could generously compare it to its contemporary Metroid Prime - while Prime has a longer story and a different approach to action (less fighting, more figuring out), Halo has a similar feel of immersion to it, and that's quite good. The environments in Halo won't blow anyone away, but the game is instead driven by the player's actions and resourcefulness in dire combat situations.
Which is one reason why the latter half of the game is a slight disappointment - hordes of enemies, sometimes literally endless, often force you to run through areas instead of strategically cleaning them out. But like I said; it is nevertheless an exciting and even fulfilling game. I may even play the sequel when I can get around to it.
Progress: Finished on Normal