I presume you mean passenger figures, in which case I'd say Oxford (with 5,798,000 passengers over 2010-2011). The ORR's figures would be able to say for certain however, I currently don't have Excel however so I can't access the spreadsheets.
If you meant train movements, surely it would be any along the Paddington-Reading corridor, or by number of stops Bristol Parkway, Slough or Oxford?
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Having read the OP again, if you mean largest by general importance I'd say either Oxford or Bristol Parkway again, though that would depend on how you'd classify Bath Spa, Exeter St Davids and Plymouth.