In order of hatred.
1. Most hated (probably enough to turn me to the dark side), was the small Inova button cell light, which initially had a non-replaceable (non-rechargable too) battery. The light was disposable after running down the battery, which was only usable for several hours as far as I recall.
Even thought no longer sold, I still wouldn't purchase an Inova light, just because they suck as a company. I guess I'm not really a forgiving type of person. lol
2. Unnecessarily small lights. I'm thinking of the small Olight i1R keychain lights, with the non-replaceable Li-Ion cell. There is no reason for Olight to make it non-replaceable, when every other company can and does. I guess they wanted to shave off something like 2mm.
3. 3xCR123 lights. Oh yeah I really hate those too. Such a waste to keep these lights fed (both financially and on the environment).
I'm specifically talking to you Streamlight, with your Protac HL3.
4. Soda can lights, not only are they fugly, but their shape just doesn't just don't make sense to me, as they are too fat to pocket, and if keeping in a pack, then why not carry a slightly longer light. Plus the soda can lights don't normally come with clips, to keep in a pack anyway.