We picked up the Best Ride On Cars Polaris RZR XP 1000 12V and we're really happy with it. It looks like a real RZR, with a roll-cage style frame, chunky off-road tires, and working shock absorbers front and rear. It holds two kids side by side (up to 110 lbs total), so siblings don't have to fight over turns.
The parent remote is my favorite feature. It overrides the driver, so you can steer, stop, and pick low, normal, or high speed yourself. It also has a dedicated brake button, which is great for younger or first-time drivers. Once the kids are more confident, they can drive on their own using the high/low speed switch and forward/reverse gear switch on the dash. Top speed is about 2–4 mph, which feels fun without being scary.
The kids love the extras too: one-button start, working lights, a horn, and music buttons on the steering wheel. The 12V 10Ah battery gives a good amount of ride time per charge. A self-resetting fuse cuts power briefly if it's overloaded, then comes back on by itself, so there's no fuse to replace.
Assembly is easy. The wrenches are included, but you'll need your own Phillips screwdriver. The manual's step-by-step diagrams are easy to follow. A few tips: buy 2 AAA batteries for the remote since they aren't included, and give the battery a full charge before the first ride.