aggressive woods single track riding/racing occasional MX day - WR 250F, CRF 250, MXC 250F, 450's are getting boarderline too big/powerful for most riders in tight woods
aggressive desert riding/racing, woods riding, occasional MX day- WR450F, CRF 450, MXC 450, KLX 450, built XR 650,
Play riding woods/desert - DRZ 400/KLR 400, WR 450F, CRF 450, EXC 450, XR 650/600, TTR 250 etc.
Ride everything put away wet - XR 650/XR 400 TTR 250
Ride everything put away toweled off - WR450F KLX 450
Ride everything put away dry and clean - 250F
Ride street and dirt roads - XR-L 650R, KLR 650
Japanese is your best bet for reliability and parts availability/running cost for general riding. The european bikes generally have more expensive and harder to find parts. If you buy used the newer you get the fewer problems you will have. They all break
My bias is towards Yamaha because I have owned, raced and wrenched on them. All of the big 5 manufacturers make good machines, but all have their quirks as to what is going to break or need attention. From your description I think that a 450 would suit you best depending on your aggressiveness or riding intention. I believe that a 450 would be less maintenance than a 250F and offer greater versatility. The down side is the added height and weight. If you think there will be a 10-20% chance of pavement riding I would lean even harder towards a 450. The type of trails is what would sway me back to a 250F from there.
Good Luck