I mainly do football and prefer the ones that are easiest to find the matches for. I haven't a clue which 'league' is for which, so prefer sites that I can narrow it down to UK teams and preferably on a day to day basis. I find that Ladbrokes works for me for this reason.
I used to find Paddy easy to use but HATE the new site as I can't find anything, grrrrrrrr.
Of course I also hate the ones I am gubbed at - you know who you are, Sky, Sun and Betway! I am still going to use them though for testing out acca's, if I find any Arbs etc.