My recent touring experience:
Keys to the Kingdom.
Marceline to the Magic Kingdom. There is some overlap between the two but they are different enough to make both worthwhile.
As mentioned, KTTK includes lunch (we ate at Tomorrowland Terrace) and is about 4 1/2 hours in total. The Marceline tour is 3 hours and does not include a meal.
Both tours include a visit to the Utilidors, but Marceline includes a visit backstage inside the Haunted Mansion.
Undiscovered FutureWorld
World Showcase: Destinations Discovered
Both were very interesting and most of the WS tour happens before that part of the park is open to guests.
Backstage Magic - the big 7 1/2 hour (and pricey) tour. Lunch was at Whispering Canyon at Wilderness Lodge and stops included the Costume Design building, Central Shops (where they build and maintain just about everything required to operate WDW and one of 4 massive laundry facilities. Certain times of the year they also visit the Horticulture area where they build the topiaries but since the Flower and Garden show is currently running, everything is out on display.
This tour also includes a visit to the MK Utilidors.
Yes, it's not cheap but it is well worth it.
The two I wouldn't recommend are the Behind the Seeds tour (unlike Grumpy Dad, I found it really didn't add much that you didn't experience riding the Living With The Land boat tour) and the Gardens of the World (which is another walk around World Showcase but this time focusing on all the wonderful flowers). Yes, you're with a member of the Horticulture dept and can ask questions about specific plants but I much preferred the Destinations Discovered tour.