Depends on the type of need - the good ones tend to specialize in a particular field,e.g. human rights or EEA. If the matter is complex, I'd advise to go for a more narrow specialist or even a barrister who'll be more up to speed on recent case law.
If it's bog standard (e.g. work permit, standard ILR, etc), then just go with a top firm.
Gherson Solicitors are generally excellent and they will have people specializing in various areas. And they will be more than qualified to handle anything bog standard.
Imane Chetouni was very good when I used her years ago for a consultation - she was at Glazer Delmar then (they don't do immigration work anymore I understand) but is at PwC now.