I have a buddy who is a full-time bicycle mechanic, and his far and away top two:

--servicing internally routed dropper posts
--6 month service on tri-bikes.

Once a dropper post has been mounted and the cable cut to length, getting it out and back in is apparently a nightmare. Tri-bikes are invariably covered with multicolored crust and lots of rust, from sweat, spilled sports drink, and best of all... used sports drink.

After hearing of this, nothing I have to put up with on my own bike comes close. Wipe it down after each ride, try to keep the drivetrain clean-ish, and I usually end up only doing a teardown 'n clean once every 4-6 months.