I like Gleim, but many are good.
When I studied for IFR, I used Jeppesen. That was before online courses, in the late 90's. I bought their books and studied on my own. When I was ready, I found an instructor to sign me off to take the written exam and aced it.
When it came to my commercial certificate a couple of years ago I went to them first but they wouldn't reply to e-mails or answer any questions. I still got their instrument/commercial book, but ended up using Gleim to study. I first got a sample lesson from them. They then called and really wanted to have my business. I was happy with their product. At the end, I ended up needing more time between passing the test and taking my checkride. I wanted to keep using their test prep software to prepare for the oral exam. They generously extended my access twice. I was ready to pay, but they gave it to me for free. I aced the written exam and it was helpful for my checkride oral exam.
I do like the Jeppesen book for reference.