I use anything and everything I can get my hands on.
I’ve got a set of each edition of Space Hulk, two sets of Tyranid Attack, lots of the D&D tilesets (the generic outdoors and cave ones work really well, and if you don’t mind stonework floors some of the basic sets work well as well). I also have some of the Gale Force 9 vinyl D&D maps. They do have some of what you might call ‘archaic’ details for a
40K setting (the odd bow and arrow or spear, sometimes ‘chests’ and other things that don’t quite fit) but those are usually easily overlooked. I have a number of the Paizo
flip-mats, and they’re great, especially the outdoor and cathedral ones. I recently got the graveyard one as well, for when my group enters the graveyard that will be next to the Adeptus Ministorum cathedral they’ll need to visit.
Maps of Mastery and a few others have made maps for the Star Wars miniatures and other various Clix games, so those tend to work well (everything from Tatooine sands through to inside space stations). Plus regular
40K works just as well, using a scale of 1 inch = 1 metre or 1 inch = 2 metres.
But if you want something that will work for anything
40K related, then you’re going to have to track down two copies (yes, two copies) of the Fantasy Flight Games produced DOOM board game. The tiles in that are out of this world. The rooms are big, and the corridors are all two squares wide (unlike Hulk tiles that are only 1 square wide) and they’re wonderfully sci-fi. And it comes with a lot of doors. Why two sets? Because you do get some odd-numbers of tiles, and two sets will give you endless variety. There are even computer programs you can use with the DOOM tiles scanned in to make the layouts before using the pieces.
So yeah, DOOM tiles. I’ve never played the actual DOOM board game, but I do not regret buying both sets. I have got more than my money’s worth out of them.