Best bang for the buck is definitely a fast single HDD.
Some of the newer 7200HDDs are nearly as fast as the 10K drives and 1/2 the cost.
Enterprise SAS (Serial attached SCSI) 15K drives are even more expensive when you throw in the controller card.
RAID0 gives you a relatively small increase in overall performance for the higher cost of extra HDD(s).
Depends on what you mean by "best bang for the buck". Are you looking for the best performance per dollar or the highest capacity per dollar? For the former - SSDs are the best. For the latter, hard drives are.
The most important thing to understand when talking about disk performance is that there are different ways to measure it, and you have to understand what they are and how they relate to the type of performance you are trying to evaluate:
Transfer rate - how quickly a long stream of data can be moved to or from the drive. Important for copying large files or running programs that have to access large files (ie, Photoshop reading RAW picture files, video editing, etc.). The best SSDs can have faster transfer rates than hard drives, especially for reads, but hard drives can be competitive in RAID configurations.
Access Time (also known as "Latency" - how much time it takes from the time an I/O is requested until the data is delivered. Important when you need to read a LOT of files, such as when booting the system, loading applications, or running programs such as a browser which use files for caching. SSDs are about 100X faster than hard drives for latency, and using hard drives in RAID doesn't improve the latency by much, if at all.
Concurrent I/Os - how many I/Os can be handled simultaneously. Important when multiple users are accessing a server, when you're running multiple programs that access the same disk at once (i.e., virus scanning). RAID, either for hard drives or SSDs, can improve concurrent I/O.
People have a habit of focusing on one or another of these metrics which leads to conflicting advice and arguments...