Church Hill, back of Benfleet (well Thundersley really) - short, but a sharp shock. Ditto the hill in Maldon town centre.
The climb out of Leigh on Sea station going East or West are short but they are hills, I suppose.
There's a couple of testing climbs coming from Laindon Station going East towards Langdon hills and cross country to Basildon Hospital that get the Old Ticker going. Back up the hill from the A13 to Basildon Hospital is also pretty good, but not great from a traffic perspective.
Then there's the slow tortuous climb (tortuous for me) up Noak Hill into Billericay. Not too steep but long enough to test you a little bit more than what is normal for Essex.