Join Date: Apr 2008
Location: Alberta, Canada
I'm going to have to (And YES - I will be biased!) with the Arabians, followed by the Morgans.
Arabians, especially, due to the genetic make up. They have larger hearts which moves blood through the body faster, allowing them to heat up or cool down much more efficient than other breeds. It's why the can be the last horse to arrive at a Endurance check stop, and the first to leave. They are very efficient for controlling their body temperatures.
Aside from that, I think they are the most intelligent, sensitive, reliable, and versatile horses around. It's why we've got National Champions in Western Pleasure, Side Saddle, Reining, Endurance, Cutting, English Pleasure, Working Cow, Dressage, and/or Native Costume. There isn't much they can't do, and there isn't much they can't do well. ;)
Morgans I second because they are one of the best breed's I've ever had the pleasure of raising and training. Great minds, great ability, athletic like there's no tomorrow, and they got a lot of heart.
And they cross REALLY well with Arabians. ;)
That's just my opinion. Everyone's will vary depending on what breed has worked the best for them, I believe.