Maybe I'm just odd, but I like parts of all 5 Acts, dislike parts of all 5 as well.
Act 1 - I like everything, really. The problem is that I've played it too much due to having entirely too many characters I'll play and lose interest in at some point in Normal. It's always fun in NM and Hell, but boring as can be in Normal, except after one of my many long hiatuses from the game. And I'm sick of having to spend 10 minutes hunting down that one stray fallen every time I do the DoE.
Act 2 - I'd like all of it were the parts outside of town not so dang big. Being primarily a melee type, though, I often get sick of skeleton and unraveller boss packs in Hell, though. This is balanced out by my undying love for the Arcane Sanctuary, though.
Act 3 - I'm in the minority that likes nearly everything about this act. I don't mind running around the jungles, and I've never been bored in Kurast or Travincal. Only negative is the Kurast Sewers, just because it always takes me forever to find the stairs down to the second level.
Act 4 - Usually my least favorite act, but still fun. It can be a total chore to play it if I get the wrong mix of monsters, though. Stranglers + Gloams is a good example, but even worse is the Unholy Trio of Blood Maggots + Vile Mothers + Corpse Spitters in the RoF. The first two are just tedious and usually expensive for a melee character due to repairs, while still leaving an infinite supply of ammo for the latter. Any other mix is fine
Act 5 - Another that's a lot of fun, just too spread out. This is the act that I try to spend the least time in, usually just finding waypoints and completing quests ASAP; it's just too dang big to spend the time to do my preferred full clears. And like most sane individuals, I see no reason to go after Nihlathak beyond Normal.
