What version of Ruby and Rails would you use to start a new project today?

Rails version 3.2.11

(Today is 2016, Jan 28) The correct answer is usually the stable major version from at least 6 months ago.Rails 5 is still in Beta, so I would not recommend it unless you know what you're doing, and if you're not sure, don't go to Edge. If you want Action Cable (the major new feature), you can build similar behavior using services such as http://Pusher.com.There is no doubt: choose Rails 4.2.x (x being the most current minor). The entire ecosystem is matured around it, the most important gems work with it (Devise, ActiveAdmin, Refile, etc). Even when Rails 5 is officially released, it will take at least half a year for the major libraries in the ecosystem to become compatible, and even then you will have to drop a few gems as they will not be upgraded.In the case of Ruby, you can stick with the latest 2.3.x series. There shouldn't be anything that breaks backward compatibility in a major way at least until the 3.x series comes up, and that's still a couple of years away. The newest Ruby has better performance, memory management, security patches, etc.On the other hand, you should not be below Rails 3.2 and Ruby 2.0, at least. Anything below that is a security hazard. There won't be any security patches for versions below.Those are the general rules.

Because your project may take several months to get to a public version, it may be interesting to learn Rails 5 and familiarize yourself with the new tools it introduces. Also, some Rails concepts are easier in Rails 5 than 4 (example: you can use 'rails' command line for a lots of things that were specific to 'rake' before).As for Ruby you can use any 2.x version you want, it should not make a big difference. So you can't go wrong with the latest stable one.

I would use Ruby 2.3.0 and Rails 4.2.5.1. Oh, did I write I would? Sorry: I do use this. I just upgraded our current Rails project to these two versions this morning (2016-01-26). I think it is to early to use Rails 5 in production. I don't think upgrading later will be a huge issue. A day of work and it will be fine (assuming your application is tested properly)

Today? I'd use 4.2.5.1 and 2.2.2, but it also depends on your exact specifications. If you don't care about certain dependencies breaking and such, or a ton of gem support, there's not much holding you back from Rails 5 beta in a true greenfield applicationThe good new is: is that the upgrade path from 4 to 5 is probably going to be as difficult as 3 to 4 is, but not 2 to 3. If you need a consistent base, Rails 3.2 is probably your best bet, as it's continuing to get LTS. That said, I'm pretty sure 3.2 works very well under Ruby 2 to 2.2. 1.9.3 has been EOL'd last year (although I think Red Hat still supports it independently). 2.0 is mostly a drop in replacement of 1.9.3 and should have a longer support life. Unfortunately I don't know the current length of support for Ruby 2 versions.