My terrain collections is... scattershot. It's generally just enough to accommodate most games, albeit with a bit of squinting and hand-waving.
Recently, though, I have been on a drive to bulk up my options, with a view to consistency, and now have enough to build a small rural village or farmstead that serves most purposes from Medieval to Fantasy, ruins in the same style, and, recently, even some trees! What I didn't really have were hills (aside from one old GW hill I think I inherited from a friend). Perhaps this is down to living in middle of the Thames Valley, but all my games were taking place on a similarly level playing field.
I decided that it was time to rectify that omission, so I snagged a few of Gale Force 9's 'Battlefield in a Box' rocky hills and escarpments. I am a big fan of the GF9 terrain and have more than a few pieces in my collection. As always, these were largely usable straight out of the box, though the application of the supplied flocks (two different types, which is a really smart inclusion!) and a spray of PVA glue to fix them really added to their impact.
Simple, quick, and effective - exactly what I want from my terrain! If I had any complaint it's that the two larger pieces don't actually sit especially flat - there's a noticeable gap between hill and table on some edges. Unlikely to be an issue in-game, but still a little annoying now that I've noticed it!
With luck, they'll see their first games of Rangers of Shadow Deep very soon...