Of course, what constitutes "worst" is a difficult question.
Signetics made the 2650, a nice processor with a highly regular architecture with a condition code register. After every arithmetic operation including loads and stores the ALU updated the condition code register.
The National 32032 processor was a wonderful part with a clarity of design that made it a great choice for a workhorse processor. Unix running the machine was stable and efficient except that every few weeks there would be disastrous crash. With a tremendous amount of effort the source of the problem was found: a race condition in the interrupt control logic that returned from the wrong stack and scribbled over memory.
The Intel i860 exposed the internal computational pipeline to the programmer. Context switching was complicated by the conflict of real-time operating performance requirements and a deep pipeline with no way to grab the context and drain the pipeline. Eventually a dedicated team got a Unix OS running on the part, but it peformed poorly.
The Maspar MP-1 was a SIMD machine. It was cool to test new library functions by seeing if, say, sqrt(x)*sqrt(x)==x for all floating point numbers. Customers wanted the Maspar machine to be timeshared, but the architecture made it difficult to do since the CPU state was very large and memory was not mapped.
Intel's 8048 (and simplified versions like the 8021 and enhanced versions like the 8051) did not perform as well in terms of speed or code size as many of the competing micro controllers. The competition offered very simple asymmetric complex architectures which could be programmed (possibly with external hardware assists) to accomplish embedded tasks with significant effort and several days or weeks of effort. The Intel part was not quite as efficient in memory use and speed, but could be programmed in an afternoon. And another engineer/programmer could look at the code and understand it without much deep thought.
The Motorola 68000 was a wonderful machine with a clear instruction set. But the original 68000 could not support virtual memory.
There have been all sorts of different architectures tried which seen strange today but came about because the architecture was thought to provide an engineering solution to an immediate problem. There was a time when register machines were thought to be a bad architecture, far inferior to a simple stack architecture.