Back to Rappan Athuk. What's your favorite floor/area? As I mentioned before, I read the first area and didn't care for it. I'd love to give Rappan a second chance, but since it's a billion pages, I'd appreciate a tip on an especially good section to check out.
That's tough. Sometime what reads well is doesn't play as well as you would think and areas that seem boring shine in play. Also, whether you play with theater of the mind or with battlemaps and VTTs will change the experience.
For VTT, the bloodways was an interesting challenge and could be ripped out of RA and used as dungeon encounter in any campaign. It really lends itself for VTT play, but I put A LOT of time into prepping it. I used Foundry, had environmental visual and vision effects to capture the blood mists, I used Multilevel Tiles module to handle both teleportal traps and to move tokens to the right spots when their is movement from one map to another. There are some interesting puzzles and encounters spread throughout it. All in all, I made for a nice multi-session challenge. For ToTM play, they have alternate rules for running it without the maps, other than a few set piece battle/puzzle maps. Would still be fun, I think, but a very different experience.
Area 3-5 "The Lair of Scramge" is an interesting encounter that can punch above its weight in terms of character levels. Would be easy to take and use in other adventures. It takes some time to prep for in that you need to read it and understand how it is to play out. It also is helpful to have a way to give private messages one-on-one to a player. It worked really well using the private chat function of the VTT (Discord, etc. would also work well). Since we were playing remotely, the other players really didn't know what was going on, which would have been harder to pull off in an in-person game. I really liked how that one played out.
4B-22 "The Lair of the Spiker" proved far more dangerous than I would have thought for the level of the party when they encountered it. The enemy has some tricks up its sleeve that are not level inappropriate, but which can catch players off guard.
The whole Cloister of the Frog God and the dungeons below it could easily be taken out of RA and used as a separate area in any campaign that needed a cultist strong hold for low to mid level PCs.
Level 7A: The Hall of Kazleth, the Phase Minotaur King was less of an interesting area. Very maze heavy and developed some separate tools to emulate the maze experience without the grind. But the phase minotaurs are interesting and tough enemies. Certainly worth looking at and using in other settings.
Levels 9B and 9C, the Well of Agememnon was a fun few sessions. Good old-school dungeon exploration, puzzle solving, deadly environmental threats, battle encounters are average, though the bid bad of that level was fun to prep for and can be quite deadly if the players are underleveled or under prepared. There is at least one area where poor rolls on skill checks could lead to a TPK, which might not be something also groups would be happy with.
I also am also fond of the several levels that make up Greznek, the subterranean town of orcus-worshipping goblins. It has special social interaction rules and is one areas where the social pillar can take center stage. It is not somewhere you can go in with guns blazing until at very high levels, and even then, a DM could run it so that it will wreck even a high-level group of PCs' day. It has some fun encounters and NPCs. It also connects to many other areas in RA and became a place the party kept returning to throughout most of the campaign.
I could go on. But these are some that come top of mind that can easily be ported to other settings and campaigns. But, frankly, most of the areas can be ripped out of RA and pasted into other settings. You need a fire-planesque salamander encouter for higher levels, use level 10 (The Lava Pits). You want a mini dungeon with a conan / skull island / jungle book King Louie flavor, tear out the Steam Jungle (level 8B). You want a cultist shrine encounter, tear out any of the three Shrines of Power (Level 4, Level 9, and Level 14). Rappan Athuk can be fun to run as a campaign or just as useful as a resource for some interesting mini-dungeons and encounters.