What development methodologies are you currently using?
What are some of the benefits over older methodologies that you are seeing?
+1 for Agile development, TDD and Extreme
Having seen many methodologies come and go the best development methodology is the one you follow. Almost any methodology will get you to your desired end-point if you put the methodology in practice. Almost all methodologies will fail if you don't comit to using them.
So I will answer your question by saying that I think RUP is still the best overall methodology.
It depends on the people you have and their experiences, expertise. Also you need to consider how many people are in your team. If you are below 20 developers then may be an agile methodology is good for you. However, you should also consider you need a good population of decent developers in your team if you are going to apply agile. In contrary, more formal methodologies are good when you have a larger population of developers, most of which are juniors.
When you are choosing methodology you have to keep in mind the following factors
1) Requirements (whether they are complex, will be changed so soon by clients, whether clients themself sure about thier requirement)
2) Delivery time you have
3) Resources ( whether they have technical competency in the technology which you are going to use)
If you are sure that Requirements will not be changed, Clients are sure about their requirements, lacking technically sounded developers, having enough time to deliver, better look for Waterfall approach (Not Agile methodology like Scrum, XP, etc.,)
On the other hand if you have changing requirements, Clients want to see the result(Partly) so soon, have some decent resources, better to go with Agile (Scrum or XP)