What do you mean "in the nex couple of years"?
If you mean who will have the best stats in the majors in the NEXT TWO YEARS, then I see Hansen as your answer because hes likely the closer next year and partly this year.
For future its tough to tell for me: Lester has great potential as a hard throwing southpaw, Zumayala has control issues but throws nearly 100 mph (although Capellan did that and where is he now?), Pelfrey showed very good stuff in his recent debut but he just signed from college and hasn't really adjusted, Diamond needs to be traded, he has very good stuff but the ballpark will deminish his career.
Sean Tracey has my apologies, we all know Ozzie Guillen is an idiot. I'm rooting for you!