I think that what people refer to as a top boy is one who: learns well and learns often; is a ben torah in and out of the bais medrash; has good middos; comes from a good family; has the right balance of personality (really serious about life, but also knows how to have a good time, etc); is probably going to stay in long term learning….
this is not necessarily a list that I would consider “top”, but what I think people refer to when they use that term.
personally, i hate the word “top” in reference to good boys who will be in learning for a while. as many others said here, middos are the most important thing when looking for a spouse. in my very humble opinion, it is not good middos to feel entitled to and demand from the girls side financial support–and expect to get it!!–just because someone is “top” in his learning. the sense of entitlement that sometimes accompanies guys who think that just because they are in yeshiva and know how to learn gives them the right to place all kinds of demands on the girls….that is not good middos and takes them off the “top” pedestal real fast IMO.
also, it is never a good idea to use a term like “top” or “best” because it takes away an important point: when looking for a spouse, one is looking for someone who will be best for THEM. not someone who is “top” by universal standards. this is a really big problem because we have people searching for illusions, and they think they sometimes get the “best/top/” out there, but whatever that is is totally wrong for them.