Firstly, let me say that you don't need any of these machines to develop for iOS. You can get much, much older versions and still happily do iOS development on them. Heck, I used a Mac Pro 1.1 until late 2015 for all my development and design work and I could still use this machine and get work done in a relatively decent fashion.
Anyway, I'd rule out the 2015 MBP, not because it isn't capable, but because you don't like the machine in the first place. So, why put it on the list, unless you felt you could convince yourself it's a machine you'd be happy with in the long term? It's just too expensive a machine that one would want to have buyers' remorse over.
And with that said, I say go for the latest machine. You have saved up enough money for this computer. It is an investment and my impression is that you seem to be favouring this MPB model anyway. Yes, it's the most expensive, but you have the money, it's the device that would make you the happiest, and it's also the most powerful machine to boot. Therefore, I say, do it!