If you mean atm, then I honestly think he's the 3rd behind Djokovic and Nadal. And I'm not trolling. Murray is in a minor slump. He did win Miami, but didn't play anything special and got destroyed by Wawa in MC, plus Delpo beat him just few weeks ago. Ferrer is injured, he's out of the discussion. Federer hasn't shown anything this year. Only thing worth mentioning is his win over Tsonga in AO, but even there he wasn't that great. Of course, I expect Fed to pick up and play better rather soon, but ATM, judging by their performances after AO, Delpo is the 3rd best player in my eyes and the 3rd favorite for Madrid (as the next big tournament) and RG (as the next Slam). Doesn't mean that all won't change as soon as after Madrid or Rome, tho.
P.S. Before some Murray fan starts nagging me, I also took into consideration that we are now in the clay court season, hence Delpo gets little bonus points.
P.S.S. Him being the 3rd best player atm, doesn't mean he should be ranked #3.
I mean in general. If a big tournament comes around, I believe Del Potro will always be at worst the fifth most likely man to win it/reach the final. On clay, he might (I emphasize the 'might') be ahead of Murray, but in general he definitely isn't. He most definitely is ahead of the rest of the field though.
He's not been quite top 4 since his return, but he's definitely the most immediate threat to them, at least a step above Berdych/Tsonga - and let's not even talk about Ferrer.