I'll restrict my nominees to pre-1995 stuff because that is mostly what I have:
For HP, they had a period where they decided to compete in the low-end market. For example, the 3435A, 3465A and 3466A are 3-1/2 or 4-1/2 digit DMMs that are cheaply built and not worthy of HP's state-of-the-art reputation. Similarly, the 5300A and 5300B are "mainframes" that accept a variety of plug-ins, mostly counters. Again, they are low-end stuff and if you want a counter, then buy a real one. The HP (Germany) 811x pulse and function generators are very nice - when they work. Almost everyone I see offered on eBay is throwing errors indicative of faulty HP-custom ICs that are, of course, unobtainable. Finally, you would think when HP developed their 6-1/2 digit DMM HP 3457A, it would be better than the preceding 3455A and 3456A. But I find the 3457A slow, noisy, hard to see the LCD display, and generally a "dog."
For Tektronix, their stuff seems electrically excellent. But mechanically, that is another story. Repair and maintenance of some of their earlier scopes (the 7000 series or the 22xx/24xx series) is a chore. Cam switches, endless brackets and screws, stuff packed into the chassis like sardines, etc. And I am not a big fan of the 500-series mainframe and plug-ins. Although I suppose it is nice to have a 6-bay mainframe packed with a variety of plug-ins, I find the individual plug-ins to be somewhat lacking in performance and I would rather use a dedicated piece.
Just my opinion...