Washing the bike. Living in an apartment means putting it in the shower-tub. The whole process is so awkward and laborious, and then there's all the post clean-up clean-up. Meanwhile, I've found that sometimes, all that's really necessary is a wipe-down, and I was happily surprised at how good a job just a soft, dry cloth did. I credit that in part to the fact that I'd applied some spray polish (like Pledge) before taking the bike out.
I don't mind cleaning the cassette too much, but the crank set and derailleurs are just so awkward....
Second least favorite: changing tires. I'm incompetent - always working up a sweat, hurting my hands. But I must say, I feel like a champ once I've done it.
My fingers are too clumsy and my eyesight is too poor to do anything dealing with tiny screws, so derailleur adjustments are out of the question. I tried it a couple of times and only succeeded in making things worse.