The OEM's are Dillon, Hornady, Lee, and RCBS. Per the internet experts at The NYT Lee sells 3x as many as Dillon, Hornady, & RCBS combined.
Dillon is typically stated as the best so I will compare others to their design.
When you watch YouTube Channels you observe things. A long time Hornady LNL guru abandoned his Hornady hear in lieu of sending it to Hornady to have it repaired under warranty for primer bellow flush issues. He has replaced his (1) Hornady LNL with 5 or maybe 6 Dillon Progressive Presses. He is very happy having dedicated Dillions to his calibers and loads instead of caliber changes.
There are exceptions to buying a Dillon for each load. These are resolved thru complete toolhead assemblies and dedicated primer size presses. Cost is significant. A rare few operate (1) Dillon 550 with (1) powder measure and multiple toolheadS of dies only.
Why are there so many upgrades for Dillon Presses ? Why are they needed. If the press needs that many upgrades to meet the expectations of the market place you have to add these to the cost of purchase.
When you watch Hornady LNL on YouTube you see the owner has (1) one. Even in forums people own (1) typically. So evidently those that buy this press load multiple calibers on their press.
Are there upgrades other than a light for The Hornady LNL ? It seems they are run stock. No after market fixes for a very old press design.
The RCBS ProChucker 5 &7 are great designs with poor marketing. It is like RCBS does not want to sell them. You can find more Bigfoot sightings than ProChucker videos. Nobody can evaluate this press against other brands. As far as I can see, no after market fixes.
Lee sells many Loadmasters. Many videos exist showing the press works, how to get it to work, and how to correctly set it up. It is as if Lee does not want people that cannot figure out the directions to use the press. The LM is a very old press design. The current version has factory improvements to the Shell plates, timing, and priming system.
The Dillon 6/750 is virtually unusable without a case feeder. The press " as shown " on the Dillon Website at this time is over $1300. This will give you 800 per hour. This places it in a production level all its own. Add a bullet feeder and 1,000 is doable per many reports.
The Dillon 550, basic Hornady LNL, and Lee Loadmaster are in the 4-500 per hour range. Yes, the LNL can be configured as the Hornady Ammo plant to rival the Dillon 800-1000 per hour for the same price. The other presses can reliably be used as a hand/ manual fed case & bullet press.
Nobody really knows the RCBS production output without case & bullet feeders. I guess the 4-500 basic and 800-1000 automated output numbers would hold.
The warranties are similar. They all have a clause to require you to send, at your expense, the part needing replaced for a free one shipped to you. Lee only gives a (2) year warranty like Dillon does on the 1100. After (2) years Lee will charge you shipping. With computers, people that routinely wreck their press get treated differently.
If you load, time at press not prep, (1) hour per week at 500 per hour you make 25,000 per year. The same time on higher output press is 50,000. So, honestly you need to honestly define your need. Many progressive presses are chosen based on time available to reload. Others are chosen to maximize time away from wives...
So while every brand has zealots they all have weakness. Dillion was the most likely to come out of the box tuned & ready to run. They changed from doing as much factory set up & verification on 550 & 750. The caliber chosen shell plates are not installed and 1st time users are having issues. The other brands are known to require mechanical aptitude, some more & some less, to set up. So you must place a monetary value on your time.
The best press for you is different than the best press for me. Many Dillon customers get talked down to a 550 by the technician on the phone. Like wise Lee states in print not to get a LM unless you shoot 500+ of that specific caliber each week. The take away is simple is often better. If your press is down waiting for free parts for (1 or 2) weeks you gained little in higher output.
Just have fun and do not form an ideology and dogmatic arguments about a brand as being best !!!