Have all the recent big technical sets. The class tractor is good but I got bored of it pretty soon in the build. If you have loads of spare parts you could design your own attachments which is fun.
The bucket wheel excavator is interesting to build and huge which has appeal. Definitely recommend it.
My recent favourite has been the crawler crane though, a good well designed set. And also like the Volvo wheeled excavator.
The Mercedes Arcos was also a good build.
Would say get the BWE, crawler crane, or Arcos. Obviously big price difference so with crawler I would also recommend the volvo as an additional.