You probably won't be dissappointed in any of the choices. That being said, I've never ridden a KTM or a Tenere.
One thing to keep in mind is maintenance and if you have all 3 dealers in your immediate area. If a dealer servicing one of the bikes isn't close to you, then I would definitely take that into consideration. Having someone close by is just handy.
In my estimation, the BMW has proven itself. They are the standard by which the other bikes are measured. The KTM has some gremlins from what I've seen and read. That being said, if you're into offroad riding, that might be the way to go. I don't know much about the Tenere's. To me, they're a bit new. I bought a newly designed Yamaha WR450F and wished I had waited a few years for them to fix a few issues with it.
I went from a 2004 FLSTFI to a R1200GS. When I test rode the BMW for the first time I remember thinking to myself "where has this bike been my whole life????" It was quite a change, and I still love the bike. It's light years ahead of the harley I owned.
I chose a GSA because of the gas tank size. It's remarkable to me that I can go the same distance on my GSA as my truck before a fill-up. It's one less thing to concern yourself with and I'm already programmed to start looking for gas after a certain amount of riding, either in my truck or on my bike.
With stock tires, I've done some trail riding on the big GSA. It does surprisingly well. It feels very well balanced to me, but it is a big heavy bike. I did some sand also. I lost front wheel traction a few times in that environment.
At 6'4", I would think the taller GSA would suit you. It's considered tall by some, but at 6' it doesn't bother me at all. The dirt bikes I have sit taller. Whenever I've ridden those for awhile and get back on the GS, it feels squat to me.
I've run into quite a few guys that have transitioned from Harley to BMW. More than you probably imagine. I spoke to a local dealer and he guessed that most of his sales fell into that category.
Whatevery you choose, it'll be a fun ride! Good luck!
2011 1200 GSA
1970 Triumph Bonneville