The worst is easy, the BC-350. It was the first alpha-numeric scanner. The trouble is that is overheated. I sent mine back a couple of times for the same problem. Finally, Bearcat offered free BC-300's to resolve the problems. The BC-300 was not alpha-numeric and eventually the digital display deteriorated and became invisible. That is a common problem for a lot of scanners, especially the older ones. The LCD screens don't have nearly the same rate of similar failure.
Another disappointment was the PRO-2052. It claimed to be the first computer programmable scanner, but the "software" didn't have a memory and you could only use a computer as sort of a control panel. I had to wait and buy a PRO-92 to get a computer programmable scanner. I loved it. The PRO-96 was my first digital scanner. I was visiting family in L.A. quite frequently at the time so being able to scan the LAPD was important to me. However, the local trunked systems that started out as Type 1's went to Phase 2's so the digital feature didn't help anymore. Mammoth/June Mountains went DMR Phase 2 and Southern California Edison went Phase 2 trunked about 4-5 years ago so it necessitated a 325P2 as both are important for me. Electric utility listening is essential as we often get system outages during snowstorms and less often during high fire dangers caused by major windstorms during the rest of the year, most often in the fall.
The best, for older scanners, the 760's. They could be modified to include to use PL tones and another to significantly increase the scanner speed. My GRE scanners are all good, in fact excellent. The PRO-92's, the PRO 96, the PSR500's and 600's are all excellent scanners. The software from Starrsoft is excellent. The developer of it had something to do with the firmware of the PSR's and PRO-96's so it could not be beat. I live in a rural area and don't have a lot of the newer Phase 2 systems so they are my daily workhorses.
My newer scanner is the BC-325P2 which is an OK scanner, but having only two AA batteries is a significant disappointment. I've heard the SDS100's and SDS200's are not sensitive on the VHF High band, which is the majority of the frequency use in my rural area so I'm not interested in them at all. I'll stick with my PSR's.