General rule - you should always use latest minor (.x) release of Unity.
Unity 3.4.x - was tested with Xcode 3.2.6 and Xcode 4.2. Unity 3.4.x won't receive any updates.
Unity 3.5.x - should work fine with Xcode 4.2 / 4.3 / 4.4 / 4.5 / 4.5.2 / 4.6 / 5.0 (except Build&Run). We strongly recommend upgrading to Unity 3.5.7.
Unity 4.5.x - should work fine with Xcode 5.0. Xcode 5.1
Unity 4.5.4p1 - adds support for Xcode 6 and iPhone6/6+. Build can be downloaded at...
Recommended Xcode versions
Unity 3.4.x - was tested with Xcode 3.2.6 and Xcode 4.2. Unity 3.4.x won't receive any updates.
Unity 3.5.x - should work fine with Xcode 4.2 / 4.3 / 4.4 / 4.5 / 4.5.2 / 4.6 / 5.0 (except Build&Run). We strongly recommend upgrading to Unity 3.5.7.
Unity 4.5.x - should work fine with Xcode 5.0. Xcode 5.1
Unity 4.5.4p1 - adds support for Xcode 6 and iPhone6/6+. Build can be downloaded at...
Recommended Xcode versions