The Obvious answer is Leeds, certainly when you take into account the immediately continuous Urban area of Bradford, Wakefield, Dewsbury/Ossett and Wharfedale.
The first thing to do would be to open up more stations & electrify the existing heavy rail lines, so places like:
Armley Canal Road
Kirkstall Abbey
Kirkstall Forge
Calverley & Rodley
Apperley Bridge
Idle
New Wortley
Armley Moor
Stanningley
Laisterdyke
Low Wortley
White Rose Centre
Batley Carr
Elland Road
Beeston
East Ardesley
Wrenthorpe
(all on the Airedale, Caldervale & Huddersfield lines) had services again. You could then 'fill in' with lines such as:
Leeds-Bradford: via Armley Road, Stanningley Road, Bramley Station, Pudsey, Tyersal, Bowling (using old rail alignment between Pudsey & Bowling)
Leeds-Dewsbury: via Old LNWR viaduct, Gelderd Road, Ring Road Beeston, Churwell, Morley Queen Street, Tingley, Shawcross
Leeds-Wakefield: via Hunslet Lane, Belle Isle, Middleton, East Ardesley, Lofthouse Interchange P&R, Lofthouse Gate, Outwood, Newton Hill