The whole point to the Slider is that any dock will work because you simply have to slide the smaller, bottom section off to dock.
That said, if you want one which allows you to use the case fully....
I've used the Griffin Simpifi since the 3G.
What's nice about it, beside the fact that in addition to being a dock it also has a built in memory card reader and USB hub, is the dock connector has a removable insert.
You can fit it to accommodate any device. If you only own an iPhone 4 simply pick up one of the dock adapters and you're good to go.
I've fitted mine with one of the universal dock adapters that Switch Easy gives with their 3G cases. I am able to use the dock to sync/charge my iPhone 4, my iPod Touch and a 3Gs as well.