Just because I'm bored...

Best for XC:
In my opinion it's damn hard to beat a TRX450R... Dead reliable, HUGE aftermarket, some of the best ergonomics, light, and easy to ride for 2 hours straight.

Best for MX:
KTM 450/505SX if you can swing the cash. TONS of power, basically race ready out of the box. Suzuki LT-R450 otherwise, has the width, great weight balance, the FI is extremely smooth in the air with no chance of cutting out on you, and the power is silky smooth.

Best Overall:
Hard to argue with the Yamaha YFZ450. Can be made to do anything well, make great power for next to nothing, and very comfortable to ride.

My riding experience:

Honda TRX450R- Love it. Easy to ride, comfortable, dead reliable, and easily my personal favorite.

Kawaski KFX450R- Eh. The loaner Kawi I ride has aftermarket suspension, a pipe, power commander, FuelATV intake, and tires. It seems underpowered, but the power it does have is smooth throughout the RPM range. Slightly more tippy at stock width than the others. HEAVY even though it has an aluminum frame. Reverse is nice, but not my quad of choice.

Yamaha YFZ450- Cam mod, pipe, filter with the lid off, safety wire the accelerator pump, fuel screw and an adjustable needle and you have one stout quad. Probably the easiest to get power out of, and is good at everything. Not my choice out of the box suspension wise though.

KTM 450/525XC- Supposedly race ready out of the crate, good shocks, decent factory a-arms, comfortable ride position, but the power doesn't do it for me. Almost too smooth, and just feels lacking. Still a nice machine.

KTM 450/505SX- Crazy. The 505 makes nearly 70hp with minimum mods, light, nimble, GREAT suspension. If you can swing the cash it is THE machine for MX.

Suzuki LT-R450- Fun to ride, make decent power, and comfortable. I hate the stock tire setup, and the stock gearing is wayyyy too short. On 20" rears it feels 100% better. Good MX quad when nearly stock, takes a good bit of work to make it work in the woods.

Can Am DS450XXC- Amazing machine out of the box for XC. Only issues come with reliability and a bad radiator placement. Good radiator guard and good maintenance and with a little luck you have a VERY nice machine.

Yamaha YFZ450R/X- The R is an OK machine for MX. Power feels lacking, not as easy to get power out of as the old style YFZ motor. Suspension is iffy, bad upper a-arm design is at fault. The X is the same way, just made for XC. Has some of the same issues, minus the a-arm design.

Polaris Outlaw 450MXR- Decent machine. Very similar to the KTM's.