A lot of casters are really good but they tend to get pricey. Bear Dudu is strong as hell with good gear, especially physical one.
Another really strong and quite cheap build is Snake Bite Barbarian. However, from my experience the cheapest one would be Screaming Eagle barb, he has crazy single target damage basically naked but the farming speed is abysmal.
Fire Druid is pretty good at it with just the Seer set which is fairly cheap. It's usually used for a cold build but it does fire damage/-res too. Rain of Fire got buffed and I've found it really quick on bosses.
As an Example at lvl 120 I did the 2 bosses at the end of the silver city and easily killed both within 1 min.
I think this question should be asked in the following way, which class/ build does the actual hard content without requiring a lot of expensive su umo or special set ups? People citing 120 ubers as a testament to a builds strength is kind of weird because basically every character can do this pretty easily. You should be looking at stuff like the extreme ubers, high labs and trophy farming fast.
ATMG sin with set, crafted shield (ATMG in), TU hached hands or crafted claws (ATMG in), +to all skills stuff deals monsterous magic damage which comparable to expensive full physical ATMG gear