I have a standardbred, and she is the most well behaved horse I've ever known. I think this breed tends to be overlooked quite a bit, as it's hard to find much reading on them besides harness racing. But they are great at any task you hand to them. I've seen and heard of STB's excelling in Western, trail riding, English pleasure, dressage, police mounts, jumping, and more.
This breed is very willing to please, has a fun attitude, and loves to move. They have the fastest trot in the world, and really love to use it! They also have a unique look, not as elegant as the thoroughbred, but rather on the handsome side, probably thanks to the morgan blood.
Having owned an STB for a year now, it has to be my favorite breed!