Obviously a matter of opinion, but I would argue that a budget android devices (or even windows phone or whatever the heck else is being produced still) made with no frills and meant to be usable by just about anyone on any income level would better fit your description of least gimmicky smartphone.
That said I don't think much of what Apple does is a gimmick. I don't see value in the new taptic engine (yet anyway; why again do I need force feedback when I am moving around the OS as I did before without it?), but I don't know that I would even call that a gimmick. Some folks have labeled 3D touch a gimmick, though I have always found it very useful.
I guess the bottom line is that anyone can consider almost anything a gimmick if they don't feel the feature is worth it to them. The reality is I can get most, if not all, of what I do on a smartphone done on a sub $100 unit. I choose more expensive units (and this includes iPhone) partially because the build quality and partially because of the extra "gimmicks" they throw in. A great example I see is iMessage.
I'd have to agree that there are definitely no frills phones on the market that are almost entirely devoid of gimmicks, though those are as much an issue in software as hardware so even some basic phones are guilty of gimmicks.
But of the major manufacturers, in fact of the high end phones I should say due to some of the smaller makers on the scene, I do think Apple has a better track record of making things genuinely useful as opposed gimmicky.
The Taptic Engine will come into its own. Aside from it's rather obvious and basic function for the home button, there are some nice opportunities for software. Not all of them gimmicks as such, more nice additions.
Take gaming for example, force feedback has been a staple in the gaming market for over two decades now, it's gone far beyond gimmick. I'm working on a puzzle game at the moment where I'm going to make use of the Taptic Engine. Completely necessary? No, it's perfectly playable without it. But there will be at least one aspect of it where having actual physical feedback, particularly one as precise and authoritative as the Taptic Engine can provide, will make the player connect with the game in a way not possible before and it just fits, it feels right.
Yeah these things can probably be overdone, do we really need to "feel" wheels turning in iOS, no we've managed so far. But some people will like the sensation, the rest of us can turn it off.
Anyway I digress, I can't actually off the top of my head think of anything on my iPhone I'd class as a gimmick. I use most every function of it on a daily basis and find all of its features either helpful or essential. I'd say 3D Touch could be a gimmick and that the same could be achieved by a long press but that's not really true, there's far more precision in the 3D Touch system.
But it's all a matter of personal perspective and preferences and that's thankfully where we're all different. I mean I'd class a thin row of icons curved round the edge of my screen getting in the way of where I'd rest my palm or fingers as a gimmick.
Or for that matter any completely bezelless device, where the hell do I hold it or rest my fingers for gods sake, god knows I've tried using them they just feel as if user interaction has never been considered to me. But there will be millions who disagree with me.
Or a screen you can tilt to give you a 3D perspective of products you might buy, ok we all thought that was a gimmick