You're right, there are a sea of companies trying to do safaris all over Africa. I'm not sure if I can tell you which one is the best (or even what is the best?), but I can tell you my experiences.
Kenya is an amazing place, and one of the few places left where all of the big cats and big mammals are still in their natural habitat roaming free! You'll see lots of zebras and giraffes and some lions, and no matter how many times you've seen it on TV, it will still overwhelm you in real life.
I went with Natural Track Safaris
http://www.natural-track.com/kenya-tours.html and had a great time, although I couldn't say if they were the best or not.
I'm sure you'll have a great time
