Not sure what to tell you here, depends on what you're looking for. How much are you planning to spend? What features do you want? Programmable? Multi-disc changer? There's a section at the Videofitness website that kind of discusses some various models of DVD (although I'm not sure how dated it is) - I would take a look there, it may be of some help to you.
I have a JVC single disc DVD player and I'm very happy with it! It cost around 150 if I recall correctly, but you can definitely get one cheaper than that. I have always had very good luck with JVC components. I can tell you that for me, working out to DVD's is INFINITELY better than VHS... just easier to control and move about in the workouts.