I think they're all in the top 100 brands;)

You forgot the Cub Cadets. They are handy when you need a lemon around.

As for John Deere, I think there are four things that lead to their popularity.

One: In my opinion they are a high quality tractor. There have been some who question the JD tractors sold in the big box stores, but I know they have the same JD warranted as those sold by the dealer. I have 14 year old JD LT-155, that has received regular service all of its life. It runs when I need it and has never (Knock Knock) needed major repairs.

Two: JD has the service centers to handle any problem with their tractors, from my lawn tractor to the biggest they make. The good thing about this is the a dealer sees a lawn tractor as profit so you get good deals when you do need repair or service. I just bought the spring service kit for the tractor from a JD dealer and it was cheaper that buying the same parts in a discount hardware or big box store. I don't know for sure about this, but I think there are more dedicate free standing JD service centers than any other brand

Three: JD has building and servicing tractors for nearly 180 years. They are well thought of in the agricultural and construction communities which flows over to the lawn tractor community.

Four: JD markets their tractors like Harley Davidson markets their bikes. When you go into a JD there is something for everyone. If you are looking for a tractor of any size there is someone there who knows the product you are buying. (This goes for service as well), If you are looking for clothing, the stores will sell you all types of JD branded clothing and hats. If you are a child there are all kinds of toys from the motorized child vehicles to a full line of small indoor toys.

As part of your report, you should find some one with a JD and find out how much fun it is to drive a small tractor. There is one of the prime rule of owning a small tractor, "Don't let your wife know how much fun they are or you will never get it back."

Last year I finally gave up my 1974 Dynamark lawn mower. I purchased a John Deere replacement and here are my reasons:
1) Readily available in several places locally.
2) Reliability
3) Parts are easy to get and affordable
4) Reliability
5) If it's not something I can fix there are several service centers locally.
6) Reliability
7) Price - not cheap but not unattainable

I have two neighbors that purchased Craftsman riding mowers. One calls his names I can't put on here. The other is usually working on his as I'm mowing. As for Toro and Husqvarna I have no reference as I don't even know anyone who owns one.
The ONLY thing I can say negative about my JD is it doesn't cut as 'pretty' as my Dynamark. Then again, nothing will as they aren't making the true floating decks anymore. My Dynamark I removed one pin to remove the deck. My JD takes 6. If I get 30+ years out of the JD like I did the Dynamark, I'm good.

You should be aware that Craftsman LT's are manufactured by either Husqvarna or MTD. If you raise the seat and look at the model number, it will be preceded by a 3-digit code, either 247 or 917. 247 machines are made by MTD, and 917 machines are made by Husqvarna. As a general rule, the less expensive mowers are MTD (247), and the higher-end ones are Husqvarna (917). John Deere provides excellent product support, and their dealers maintain a good stock of parts. I have old Deere STX's that were discontinued back in the late 1990's, and my dealer has been able to supply any part that I've needed. Husqvarna was once Roper, AYP, and Electrolux; they produce good machines (in fact, I have a Husqvarna in addition to my Deere's.)

With tractors especially, you usually get what you pay for. The efficiency, power, reliability, usability, and headroom of all the parts combined is what matters. By headroom, I mean the ability to exceed expectations.

Also, tractors like auto's, have an exclusivity factor for some people and are willing to pay for it.