SNA is closest and most convenient. It's about 20 minutes away.
Long Beach is next closest, more like a half-hour.
LAX and Ontario are both within around an hour.
As stated before, San Diego is 1 1/2+ hours away.
(Times are estimates, and can vary a lot with traffic).
I always choose SNA unless another airport is much cheaper. You also need to factor in transportation costs to and from the airport, which should be reative to the proximity to DL. I'd only consider San Diego if I was planning on renting a car and it was much cheaper, and/or I was also planning on visiting sites in San Diego too.