I don't think any TOC has been perfect, far from it, infact many of the best regarded (Anglia, Chiltern, FGE, MML, GNER) have had big problems at times. In the same respect, even the worst havn't been bad all the time (even Central had their good points). In a lot of cases, good or bad TOCs have been more down to the franchise than the operator, the "best" have typically run fairly easy routes, with good stock, good infrastructure etc, or had big improvements during the franchise.
Few TOCs have really made significant impacts on their franchises, beyond changes of name and livery, many of the changes have been down to SRA/DfT specifications (service improvements, new or "new" trains, longer trains, refurbished trains, improved stations), regulations (new trains to replace Mk1s), or just been taking advantage of infrastructure improvements (increased frequencies, shorter journey times). Now it's even more so than ever, with the DfT specifying almost everything but the name and livery (although some people think they should do that aswell). one have actually made a few changes beyond the franchise commitments (providing Acle and Lingwood with a Winter Sunday service for a start) and have also kept all of Anglia's changes that wern't included in the franchise specs (I think Norwich to Cambridge was, but I don't think the additional services on the Sherringham line were, and the additional capacity on summer saturdays to Yarmouth defintly wasn't, lucky since neither TOC has been great at actually providing it).