What do you plan to use it for. Depending on what your needs are the answer may vary. Do you want to do emulation on your phone or just simple tasks, does size matter, do you have brand or os preferences in mind, and will you be jailbreaking or rooting it?
For Performance
The LG G2 is going to be anounced august 7 and have the brand new snapdragon 800 with krait 400 cores and andreno 330 gpu. I personally am torn between getting a snapdragon 800 Phone or waiting for tegra 5. Currently no tegra 4 phones have been announced yet, and the only s800 phone is the G2 to my knowledge. Both the tegra 4 and snapdragon 800 seem to be the fastest soc phones available for android.
Mainstream
As far as android goes the galaxy s4 is a nice phone with decent performance and large market share with the snapdragon 600 chipset in the us which will deliver you adequate performance for any daily tasks and lighter emulation.
The iphone 5 is a good choice as far as apple goes, but rumors of new ones and a more closed ecosystem drove me away from getting one.
Currently I have an htc rezound on verizon and I am in full 4g coverage and can say that I have been able to play games such as battlefield 3 and chivalry with under 150 ping on my 4g.
Please let me know if you have found any errors.