Pikmin
Something that struck me early on was Pikmin's difficulty - the game mechanics are sufficiently complex to have a hefty learning curve, and there is a tendency for it to exhibit Lemmings-esque frustration with the utter idiocy of Olimar's little worker-soldiers, especially as the enemies becomes more difficult and better able to demolish large numbers of pikmin at once. So, as far as games that I really like go, this one tends slightly toward the more annoying end of the spectrum. But it's challenging, which I admire.
The complexity of the game's puzzles never grows too great (which is good, because I had serious concerns about my time limit as I went through some levels). Pikmin's "game" lies not so much in planning, but in the player's ability to quickly and decisively react to threats, shifting goal priorities in real-time. This is something I'm historically no good at (I usually prefer to throw dozens of Scouts at my problems), but I still had fun working my way through Pikmin, and look forward to picking up Pikmin 2 when I can find it.
Progress: 30 parts in 24 days