I've been using the RAM X-Grip and have been very happy with it. The Wire Cutter did a very comprehensive review of it. Its easy to use and really sturdy. It'll take a lot of different phones and sizes.
Having gone thru many phone mounts, I found one that never falls off, even in this stupid Texas humidity. Spend the $ and get this RAM mount. It is worth the money. I also got the weather tech floormats. I used to live on a gravel drive way and it was well worth it. Now that I live in the city, they are still really nice. The factory trunk liner is worth it as well. Keeps stuff from sliding around.
Only other accessory I got was a V1 with the remote display. The remote display fits perfectly next to the start button, and the cables can be hidden easily using the power supply in the glove box.
I use a RAM suction cup mount with an X-Grip. https://www.amazon.com/Ram-Mount-Suction-Universal-X-Grip/dp/B0066BWMNU
RAM X-Grip, preferably with the Twist-Lock base. It's expensive, but it'll hold damn near anything and the parts are modular, so you can piece together exactly what you need for whatever application you want.
It holds well enough to keep my S3 in an Otterbox Commuter securely fastened to my near-vertical Jeep windshield, even off the street.
I believe Amazon sells the X-Grip with that very base in one package.
Edit: Critical Linkage
I have x grip phone mount for my Nokia Lumia. Its almost the same size of Apple phones. This is one of the best thing you can ever buy.
http://www.amazon.com/gp/product/B0066BWMNU
I had checked dozens of phone mounts. This one is probably best for price. Used for firefighter/cops etc. For the price is awesome. Easily detachable and you put it anywhere you want, unlike some others. The only issues I came across (and was told) was that the screws they provided seemed smaller, but were not. You need to use a good screwdriver and some strength. That's all. If you are unsure about this take it to local shop and get it done. I tried it myself and was not sure and didn't wanna break anything. I got it done from my friend.
On a very hot summer day, if you leave your car parked in hot sun, the mount might come off. But it can be easily attached again. I would buy it again in a heartbeat. Highly recommend it. Been using for 6-7 months now. 10/10 for me.