Loathsome is a little bit strong, but it was the only word beginning with L that I could think of. But it also sums it all up quite nicely. Lockhart is simply incredibly irritating, so I suppose I wasn't alone is always wishing Harry would just hit him square in the face. But that wouldn't exactly be in character, and would take some of the weight from Hermione's exclamation in the third book: 'we attacked a teacher!' Oh well. In any case, Lockhart provided Rowling with a nice source of humour because of how thick he is, and I'll leave you with a few.
'Yes,' he said dramatically. 'Freshly caught Cornish pixies.'
'She has been Petrified,' said Dumbledore ('Ah! I thought so!' said Lockhart). 'But how I cannot say...'
'I'll make it,' Lockhart butted in... -- 'Excuse me,' said Snape icily, 'but I believe I am the potions master at this school.' There was a very awkward pause.