3 years for us and it's the perfect gap.
DC1 was old enough to help, out of nappies and getting the free childcare (to give me time with DC2) when they were little. Old enough to be out of a pushchair and not to have the stress of two babies at once. Also only had to pay for one child to go through nursery at a time.
But, 3 years is still close enough in age that they have always played together. It helps that they have the same interests and hobbies. But they truly are best friends. Even now as teenagers, they still had 3 sleepovers together over the Christmas holidays.
Also, 3 years is enough of a difference so that Uber competitive DC1 doesn't compete with DC2. I personally think if they were any closer in age it would disastrous because they would compete all the time. As it is. We just had it for major things like their SATs. DD2 beat DD1 by one sub level in one sat, and boy was DD1 not happy. Thankfully though, with three school years between them, DD1 doesn't compete too much with her younger sister because they are at different stages.
But, where I'm really seeing the benefit is now, the teenage years. I've only got one going through exams, and can focus time more on DD1 as DD2 is year 8, and doesn't need as much time / support help etc at the moment. I couldn't imagine trying to support two teenagers through GCSEs and first year A levels at the same time! Thankfully by the time DD2 is doing her GCSEs, DD1 will be at uni. And thinking further ahead, DD1 will have finished uni by the time I have got to pay for DD2.
Things I didn't consider when I had my children, but in hindsight, I'm very pleased about now.