From Wikipedia:
Breaks exceeding 147
At least five breaks in excess of 147 have been recorded. A 149 by Tony Drago in West Norwood, London in 1998 is recorded by the Guinness Book of Records as the highest in this category. In that match Drago was foul snookered and chose the brown as the free ball, to score one point. He then potted the brown again, for four more points, before potting 13 reds and 13 blacks, a red and a pink, a red and a blue, then all the colours. A 151 is reported to have been compiled by Wally West against Butch Rogers in West London's Hounslow Luciana snooker club during a club match in 1976. After Rogers fouled, West took the green as the free ball and then went on to clear the table. In October 2004, during qualifying for the UK Championship, Jamie Burnett achieved a 148 against Leo Fernandez, becoming the first player to achieve a break of more than 147 in a professional match. Jamie Cope was reported to have made a break of 155 in a practice frame in 2005.