In my opinion...
In my opinion OTTBs are the best. I've ridden several, started several, and now own one. OTTBs that made it on the track usually mean they are extremely athletic and will generally excel in any discipline you put them in.
You have to be careful though, a lot of them will come with health issues and injuries, whether it be something you can fix or not. One that I was starting for someone came off with chronic back issues, they spent more trying to fix his back than the actual horse itself. Another one that I started was dead lame for nearly a year after they bought him and again dumped a ton of money into vet bills for him, but both horses turned out the be some of the best show jumpers I've ever sat on. I just got my girl 6 months ago from someone who started her when she came off the track 3 years ago. Luckily, she had no injuries, at least non major. Except she still struggles with ulcer problems today, like many, many OTTBs do. One of the risks with that is repetitive colic. Another problem I have with her is that she is only 7 and gets tender joints sometimes. So, she gets regular joint injections, Legends injections, and is on a heavy duty joint supplement. But has long as I keep up with her joint care I don't experience any problems, even during the winter.
So I highly recommend an OTTB, as long as you know what to look for it one. As long as you understand that they are trained a little different, even on the ground, and you can handle the typical TB moods and flightiness. Even if you don't at least have someone around who does.