For me it goes like this:
#1 RoCo 1 - The Paladin and the Nightblade, the two most popular classes in our campaign. The background picks, the extended stat mods and dev points. It could be argued that this book is just as important as the core rules themselves. Lord Level spells, Maybe the best RM product ever produced.
#2 RoCo 3 - The Magus class is beloved by my players, though we do a variation on it. Also... Nether and Plasma!
#3 RoCo 2 - All the skills, + rounding out the Paladin Base Lists, and the Warrior Mage.
#4 RoCo 5 - Lots of good Arcane Lists, the essence critical table, and other goodies. I still don't get that terrible Lightning Ball chart though!
#5 Alchemy Companion - Love the alternate types of Alchemists, this one is close to RoCo #5
#6 RoCo 6 - NPC quirks, More lord level spells, good stuff.
#7 Spell Users Companion - Some really nice lists, especially useful for an Archmage but there are some things that I've never found particularly useful to the campaign and a lot more lists that seem overpowered/underpowered or out of balance.
#8 - RoCo 4 - I like some of the Enchanter Lists, we made the Houri lists available to bards and that has worked out well.
#9 - RoCo 7 - This one usually sits on the bottom of the pile, least liked and least used book in our campaign. Some of the spell lists are odd or quirky and most of the book is dedicated to Tarot cards. This seems less like a RM product and more like something I'd find on the bookshelf of an old wiccan lady I once knew.