How often are you going to be clipping? If you clip regularly and you want some really powerful really nice clippers I'd say Oster Clipmasters. They're a little heavy and pricey but excellent for heavy duty work on multiple horses. And they will last forever!
For the normal 1 horse usage I really like Oster Power Pro Ultras. They're pretty powerful, cordless, and very quiet. Plus the battery lasts quite a while.
Just remember to make sure the horse is clean before you start, use lots of oil to keep your blades lubed up, and lots of CoolLube to keep them cool!