Depends on what you're training for - Mill hill in Shoreham and the south side of Ditchling are like many of the climbs you get in road racing ie long and rolling but not very steep.
Ditchling north side tends to be too busy unless you're out there before 7am. The Dyke's good as it is long and has several different gradients. The Bostal is probably the best as it tests you severely whichever way you come from.
I'm gonna give Elm Grove a go this winter season as it is long enough (around a mile) and is pretty much the same steepish gradient all the way up. Also it is round the corner from my house and at 5.30am there won't be any traffic.