As an iOS Developer, I spend a lot of time with iPhones. I have tried dozens of cases and have a couple of favorites.

iPhone 6/6 Plus

I've tried 6 cases now for the iPhone 6 plus and 2 for the iPhone 6.  For the iPhone 6 Plus the Spigen clear slim case is the best value for the money.  It is slim and doesn't make the phone feel bigger but it does help grip.  Another important aspect is that the plastic comes out from the front of the screen far enough to make it fairly safe for drops.  However, the best case, regardless of cost is Apple's Leather Case. The leather case really helps with grip on the larger phones.  I have a black one, I wish I would've bought brown because it wears really nice. Almost like a football.

Apple Computer

I also like the Invisible Shield Glass screen protector fro the iPhone 6 plus.  With such a large screen, it's a lot easier to scratch and potentially crack.  The Invisible Shield also has a lifetime warranty so if it breaks or anything else, you get a new one.

iPhone 5s/5

The Power Support Clear Air Jacket is my favorite case. It adds little bulk to the phone and does a good job protecting your phone.  The clear looks great on the iPhone 5s in any color.  

The elago S5 Outfit MATRIX is a close second.  I have had 4 of them in different colors.  For less than $15 it's a great value and does a great job protecting your phone.  I like the Dark Gray and Orange version.