The Extreme is a good entry level kart, but in my experience is not as user friendly and fast as the OTK. It's been a few years since I worked with an Intrepid; it was okay when tuned well, but had a small tuning window.
Birel brands are pretty close to OTK performance as well.
In reality, go to your local track and see what brands are running up front and what kart shops are supporting them. If you have to ask the question "who makes the best..." you probably don't need that. The question you should ask is "Who can offer me the support I need where I race."
The eagle chassis from Comet Kart sales is very popular in Ohio and Indiana and Carson/Brad Smith that run Fremont are dealers for the chassis. Call either them or Comet for more information. Very user friendly to setup and drive.
Believe the Eagle is made in Indiana by Comet an$ it is a very good chassis.
MGM Espionage is the top chassis in the south.
With the MGM you get first class priority customer service from Paul Rice and many times other drivers ( that’s why we call it a family).
Gary Lawson from a Comet is also gives great customer service and they’ll both answer any questions you have and give any help and I site you’d need.
FB Marketplace is the best place I found for used LO206 chassis. There are a ton of Buy/Sell/Trade karting groups on there as well. Only downside, you have to sign with the devil and open a FB account.