Frequency is probably the most 'in the zone' game I can imagine, and for a few months back in 2002 I was completely obsessed with it. I'd play it in the morning before leaving for work and find my fingers absent-mindedly tapping out note sections on any available surface throughout the day.
The premise is simple and perfectly executed: The player moves along a geometric tunnel, each side, or 'lane' of which represents a musical element in a particular song - drums, bass, guitar, vocals, and so on. Spread across each lane are nodes that have to be hit on time as they pass through the player's interface at the bottom of the screen. Hitting all the nodes in a section of the song activates that instrument for the next few bars, and the player can move onto another instrument by rotating the tunnel. In this way the song is built up and score multipliers are won by activating multiple sections without breaking the chain. Missing notes not only breaks the chain and forces the player to wait for the next bar to pick it up, but energy is lost. Run out of energy and it's game over.
Things start very simply, with notes to the left, centre and right spaced out very generously, playing along to slow-tempo songs. Aside from a couple of difficulty spikes the game is structured very well, and eventually the player finds themself rattling off lighting-fast combos on the top tier of songs. This is where that zone comes in. There are moments during Frequency where your mind simply detaches itself from your fingers and lets them do the work. Somehow you find yourself getting through chains of notes you can barely follow with your eyes, and that's an amazing feeling.
Harmonix have gone on to tweak and reinvent their rhythm action template, but so far they've failed to hit the purity and perfect structure of Frequency again.