The Nexus 5, X Phone and GN3 are all rumors with not substance to back them up yet. The Nexus 5 and GN3 are Q4 phones and maybe the X Phone is Q3 phone launch so until we get real info you can only work on what is known today.
I chose the GS4, but for me its going to depend more on what Verizon offers on their GS4, and if its not enough I have to look at taking my primary phone line to AT&T and leaving my data devices on Verizon and then consider the HTC One but that isn't such a great option given HTC's record for screwing their install base over and Sense 5 looks like a 1st grader did it.
The N5 and GN3 are both in the, "we'd be super surprised to not see a release this year" category, 99.9% sure those two are real. Being fall devices, we can also be sure both will blow away the spring devices from this year and will be beaten by next year's spring devices. That's how this cycle works.
The X phone is allegedly being announced during Q2 within 60 days of the availability of the One, so waiting to see it in the mix is definitely worth it; especially since HTC is again not releasing evenly across carriers. Since it requires 36-40% of American's to switch phone carriers to use the One, I would take it out of serious contention for most in that market. That leaves the S4, or 3 other devices of which 2/3 will be available within 6 months and the 3rd should be announced within 2 months and available within 4 months of S4 availability. Of course the Nexus likely has that same issue of the One, over a 3rd of American's can't use it without switching carriers.
For users outside of the US the choices are much more interesting (all 5 devices, + Padphone, etc, etc). That's the primary reason for holding out to see the Nexus and X Phone. As both of these are likely to be released carrier free at huge discounts compared to Samsung and HTC devices, while being superior devices, it helps us break up the hold that carriers use to dictate these ridiculous restrictions on who can use which devices and how they are able to use them.
Both of the last two Nexus devices were superior to everything else that existed at the time of their release. The GNex held on for over 6 months without being beaten and the N4 only had the Note 3 to compete with (a very close race) until the One and S4 releases, 7 months later. Because we don't get other devices that hold on undisputed for that long, to me it's always worth seeing what Google brings prior to compromising with 3rd party stuff.