It's got to be a Triumph, but I must admit I'm totally biased. The 650 unit twins are marvellous, however the single carb Trophy is the more tractable machine over the Bonneville which has the name and reputation. 500cc Daytonas are cheaper and more revvy, and quite quick, but it really goes down to styling and what you like. Do a lot of research, this forum, books, my website. Spares are no problem, and readily available.
Hugh
www.thebikeshed.net