According to a wikipedia article I just found (obviously you have to take this with one, if not two heaps of salt) the top 5 are:
1. China
2. USA
3. India
4. Russia
5. North Korea
These are ranked solely by the size of the army and does not take into account spending or anything else.
Personally I would put the US on the top of the list when you take into consideration how many ships, subs, and planes they have, plus the fact that they pretty much innovate and create everything new these days.
Link to Article