Senior barristers, or in Scotland senior advocates. Usually over 10-12 years at the bar.
We have a system where in higher courts people are represented directly by lawyers but by barristers/advocates - the theory is that they have a primary duty to the court and not to the client. A lawyer will instruct a barrister/advocate to run a case - in more serious cases a QC (senior) will be used along with a junior barrister/advocate.
NB My wife qualified as an advocate in Scotland - best bit being that when they are training (mostly following a qualified advocate around for a year as an unpaid assistant) you are known as an advocates devil. The whole process of training is known as devilling and the advocate training you is your devil-master. :-)
Expensive top flight barristers* with lots of experience (10-15 years minimum) that are recommended and appointed QC by the Lord Chancellor. They take precedence over other barristers and tend to be very expensive, experienced and with a proven case history and reputation. You can tell them because they get to wear silk gowns (sometimes being elected is known as "taking the silk")
Kinda the barrister equivalent of being elected to the National Academy of Sciences or a Fellow of the Royal Society.
* Barristers are one of two types of lawyers in the UK (the other being solicitors) they do most of the court-room advocacy etc.
edit: QC = Queen's Counsel. You pretty much have to have one if you're going to plead a case in the High Court IIRC.