I have a 650R. I love it. I used to have 2 of them but I got tired of maintenancing a bike for my friends.
What kind of riding do you do? Are there mountain trails that you want to hit in the national forest? Are you just doing smooth open roads? How tall are you? What do you want for maintenance requirements?
Here are my opinions:
XR650R - great bike, low maintenance (600-mile oil change interval in manual), a little heavy in the slow stuff, pain to pick up when I crash on hill climbs. Stock form is restricted with soft supsension. Uncorked with suspension mods makes it a great bike. Seat is more comfortable than all the bikes except a 650L. Best bike for going fast over relatively smooth terrain. Very impressive power sliding capabilities. Like all XR's, this bike has an overbuilt motor that is not highly stressed. It will last a long time. The smaller 450's and 525's have high-revving, light motors that put out tremendous power for their size/weight but pay for it in longevity when ridden hard. The kick start is very friendly on this bike. I can start it cold with flip-flops on. Just kick through a few times with the decompression, get slightly past the compression part of the stroke, and give a smooth kick to the bottom of the kick-starter travel (no jabs like 2-strokes).
DRZ400 - Factory Street Legal!! Great bike, low maintenance (but a little more than XR650R), slightly lighter than XR650R, E-start, Requires uncorking tuning and probably suspension work to make it a great bike. I rode with a buddy who had one (suspension and uncorked/stock carb mods). I was impressed with it. Handled tighter trails better than the 650R yet had good street manners.
KTM 525 EXC - I have 2 buddies with these. 2007 and up are Factory Street Legal!! They need uncorking. Similar top end power to uncorked 650R. WAY lighter (about 35 lbs) than the 650R. A little squirrely on the road and high speed trails. I guess its slightly twitchy like an MX bike, but not as bad. E-start. Shorter oil change interval, factory recommended synthetic oil. Good trail suspension. Killer bike for tight trails and pretty good for fire roads. Lacks XR650R stability, but gains flickability. Super strong brakes. You need to get used to them. The stock seat is most UNCOMFORTABLE. If you are tall, you can get an enduro engineering
tall seat. I wouldn't get an older KTM. It seems that they do minor updates every year or two and are finally getting a good bike that doesn't break.
CRF450X - Great suspension for stock. Still needs uncorking to get full power. Make sure to buy a newer one that doesn't have the 3rd gear siezing problem. I think its 2006 or newer. About same weight as KTM 525exc. Frequent oil changes required. Good all-around bike, probably equal to the KTM 525 EXC in tight stuff. Not sure abuot high-speed manners.
WR450F - In my opinion, I think this bike is similar to the CRF450X but better. Like the 450x, you have to change the oil frequently. I test rode one (WR426F) and seized the motor. I don't think the owner put oil in it ever. Glad I didn't buy that one. I hear these are great trails bikes with decent high-speed manners. They require uncorking also, but have pretty good suspension like the 450x. I wouldn't expect this bike to last more than 5 years (15-20,000 miles). Like the 450X and 525 EXC, it is a high strung motor. The 450's are e-start. The 426's are kick start with a bigger gas tank. The kick-start was pretty stiff and I would recommend using boots.