What is the worse vendor you have worked with?

And why is it Oracle?
- Because Oracle is made up of laywers. They and a predictor in seek of pray in the form of BS but legal contracts. They will bill for Java, Virtualbox extension packs or whatever else they can find. It starts will a simple call to a publicly listed number and ends with them billing large companies millions because some rouge employee agreed to something.
  
  I was getting at an old Stephen Colbert interview meme from TDS which was asking a questions and giving the answer he wanted to see in the second part of the question.
  "Who was the greatest president and why was it Ronald Reagan?"

I have limited experience, but Cisco using the same part number for subsequent generations of a part is just mind boggling. Having to call your Cisco rep to demand a picture of the part they're sending because they've sent the old version twice now is just mind boggling.
And yes, I'm prepared to receive a picture of the old part, I have embraced it.

I consider myself lucky that I haven't had to deal with vendors that I hear so many people complain about like ms or knowb4.
The worse I have come across is WaveIP an Israeli radio vendor. They would absolutely refuse to action any tickets I sent in until I told our CEO who would then ask their CEO what's going on and suddenly I would get support.
This isn't me jumping the queue, this is we experience a bug and replicate the bug in our lab and put in detailed steps on how to replicate the problem in the ticket and we get nothing from the vendor apart from the 'Thank you for the ticket, here is your number' and get nothing else for 2-3 months before I brought it up with my CEO.
Some bugs were cosmetic like the management Vlan field doesn't save when using Firefox or the webui would truncate RF frequencies to a full number despite able to use half frequency steps. While others were fundamental flaws in their products like multicast packets are being converted to unicast frames causing OSPF to fail.

Broadcom is pretty bad, I've had to migrate away from products after Broadcom bought them and jacked up prices, changed licensing, etc. Generally speaking it feels like Oracle and Broadcom are the worst of the worst IMO.

Oracle, Atlassian, Microsoft, AWS, Cisco.... So many choices, that I can't pick just one!
- Customer: I'm thinking about Jira
  Me: Ahhhhh!
- That's... basically all the cloud vendors out there...

Microsoft. A TV set top box we were working on had a Microsoft OS. Opening support tickets was like shouting into a void.
- In my experience they aren't actually that worst vendor. Sure they are bad but companies like Intuit make them look great.

Juniper. Their licensing team has taken nearly a year and have not sorted out my licensing. Rip off.

Sophos. During their "edge protection is ALL the malware security you need or should have [and just happens to be what we sell now]" phase ~20 years ago.
Had a stand-up argument with one of their clowns when I worked as a security consultant. Ensured I would never do business with them. The guy was a mindless cultist before it became popular.