I'm really liking the 900cc Vulcan v-twin I bought last year. But looking back, historically, I'd have to choose the '68 Triumph 650cc Trophy that I bought while in college after getting out of the Army.
long story.
This was the second Triumph I owned having had a very-used 500cc scrambler for a short while that was a maintenance headache. The 650 Trophy needed a little work to get it going, but I had rebuilt the 500cc twice in the year I had it, so I had most of the Whitworth tools and was familiar with the engines which were the same except for the bore (heads and cylinder jugs). That 650 Trophy had belonged to a Marine who had been killed in Vietnam. It had sat for a couple of years and was being sold by a friend of his for the Marine's mother. So that bike always had a special meaning for me. I rode it around Missouri, back and forth to Texas, and then to Georgia while I was there (now here) for Army National Guard training. Finally trailering it up to Maryland where I had taken a new job. It was then ridden all over the east coast until it was damaged in a parking lot accident (outside my apartment one night while parked). It never ran right after that. I always meant to take the time to restore it, but didn't. After my move to Georgia, where I live now, I advertised it for sale along with a Triumph 650 Bonneville on a company computer bulletin board, and they were bought (for him and his son) by a fellow in Sweden who did a perfect restoration. He sent me the pictures of the restored bikes about 2 years later.