(Yamaha, Kawasaki, Suzuki, and Honda)
About the time my brother and I were in highschool we finally found some barn find old motorcycles and started the motorcycle craze.
My brother and I have wrenched on all of the major motorcycle brands, including Honda, Yamaha, Suzuki, and Kawasaki. Why - When we got the barn find motorcycles, they didn't run. Usually the motorcycles had been parked for many years for some reason and never got fixed. Over about a dozen or so motorcycles we settled on Yamaha, simply because they are the easiest to work on and require the fewest specialty tools to disassemble. Honda's were the opposite and were theleast favorite to work on.
Yamaha - We discovered for the most part that Yamaha's could be rebuilt with typical mechanic tools, a socket set and open end wrench set, a couple screw drivers and plyers, and you could rebuild the motor, often without having to pull the motor out of the frame.
Kawasaki - I owned a Kawasaki KLR 650 for about 4 years and really liked the bike, it got me into dual sport motorcycles. There was one thing I did not like on the KLR - the chain driven cam balancers. This chain driven system can break and it makes a bit of noise slapping in the case. All of my other motorcycles use a gear driven cam balancer. Kawasaki makes a good products, I just disagree with some of the engineering.
Suzuki - Suzuki's are again a good product. Artic Cat have used there engines in there snowmobiles and ATV's for years. This is the brand I have the least experience with, only wrenched on two Suzuki motors. But they were very well built and had some unique features (Fuel filter was built into the fuel pet cock)
Honda - These always seemed to require special honda tools to dissassemble, they also were the only motors that used allen and torx bolts. My brother and I frequently joke with each other how terrible it is to rebuild a Honda.
Overall, any of the japanese motorcycles are all pretty good, my advice is look at your local dealers and go with who has the best local support.
KTM - I have never liked the KTM or Husaberg's. They are fast, HP per pound they are just about unbeatable, but they require a lot of oil changes and from what I have discussed with others, they are not fun to wrench on. Getting parts for them is a bit more difficult and the used market is not there like the japanese brands.
BMW - Never really looked at BMW, cool bikes but way too $,$$$ for my liking.
China/Honda clones - I would not reccomend any of the china clone bikes, they are cheap but getting parts may be difficult.