[foxpc]

Regarding the design part: I usually find a couple of landing pages that I like and create something similar with the colors I prefer. Or buy one from ThemeForest.

Back-end: Whatever I prefer at the time. If you're building a project with RoR, it does not mean that the landing page has to be done with that. A simple PHP/NodeJS app works great since it can be much faster to build.

All in all, I try not to spend too much time on the landing page. Usually, more time is spent figuring out what's best to show on the page itself than building it.