My first bike was a '71 (I think) Yamaha 175 enduro. I bought it used from a guy at our church, paid $400 for it. I was 14 years old when I bought the bike, I lied about my age in order to get a job, (at 13), to buy a bike.
I rode the wheels off, (literally), of that 175, went everywhere on it, roads or no roads, didn't matter. I had the bike for a year before I even had a license, I was raised in backwoods Louisiana, and the local deputies didn't much care as long as you were reasonably safe,(right !)
I then purchased a used '73 model Yamaha RD350, that bike was a jet. Installed expansion chambers, jetted the carbs, loved that sound.
My best friend was riding a '72 model Kawasaki H2 750, and we sounded like a pack of hornets going down the road when we rode together, he had chambers as well. He eventually moved up to the Z1, and I bought the H2. I loved that bike to this day, even considered buying one recently, but the guy just wanted too much coin.
Got married, had kids, couldn't afford bikes for a while. Later I had a Kawasaki KZ750, a Honda CB900 Custom, "retired" again for a while, and now I have the CB1100.
My all time favorite is probably the H2, simply for the sound, and of course performance, that bike just had a personality that is unmatched to this day, (lucky I survived that bike). There are now much quicker and faster bikes, of course, but I don't think I have ever seen a bike that was just plain nasty like the old H2.
The CB1100 is my current favorite, and may well end up my all time favorite, I will let you know in a couple of years.