Absolutely line art. Thumbnails are fun, sketching is usually fun, inking is fun (different from lineart in my process), colors are relaxing... but lineart is just a pain. It's especially grating since my cats stole my wacom pen and I can't find it, and I'm stuck using this little lenovo pen, which started bugging out a while back so I get jerky lines more often than not. This means I have to be even more methodical and slow in order to get decent results. Also, lots of ctrl+z.
I think this answer changes for me every so often. I remember finding flats really tedious just year or so ago. Now I enjoy that part.