My favourite ones to build up are:
-Cooking (love how the animations change from beginner to pro, plus all the new recipes that keep coming)
-Painting (love watching them paint and some of the stuff looks really nice to hang on the walls)
-Wellness (so relaxing to see my sims practice yoga! Also love the beginners' clumsiness)
-Handiness (being able to improve objects little by little is really rewarding)
My least favourite:
-Programming (just sitting there in front of the computer for hours)
-Comedy (writing jokes on the computer and telling the same jokes to other sims gets boring quickly, unless my sim is a comedian, I just don't bother with this one. If it builds up, it just does it organically while I play, never on purpose)
-Charisma (same as comedy but swap watching your sim writing jokes on the computer with practicing a speech in front of the mirror)
