What is the best degree for future employment with an alphabet group such as DEA/FBI/CIA?

The only advice that I can provide is to get a degree in the Information Technology. It will definitely give you a job for the next 10 years. It is a high demand job for any highly qualified IT Specialist who understand a problem-solving and think out-of-a-box. You can specialize in the Cybersecurity Engineer or Information Assurance. Due to a recently OPM's PII Data Breach of over 22 Million Government Civilian personnel, Contractors and family members, there are going to get worst in the next 5 years. If you have a credential or skill sets that I mention, you don't have anything to concern to get hired. Based on my experiences as a Cyberspace Operations Planners instructor, the bad guys are always doing their best to hack into the Federal Government agencies and major corporations to steal our intellectual assets. Those agencies will hire you to defend their networks.

Probably like a lot of other endeavors, the most demanded and secure is IT or IT support (especially with a cyber-focus)

I earned my BSc in Public Administration and that, combined with military experience, was more than sufficient to be hired by any local or state law enforcement agency in California. I was actively recruited by LAPD, LA Sheriffs, and the CHP but elected to go into a career in education. In college I had a number of instructors who were retired law enforcement from local, state and federal agencies. The advice they gave to those wanting to go federal was to get a BA/BSc in a law enforcement related major like Public Admin, Criminal Justice, Psychology, or PoliSci, gain employment with a state or local agency, and then get a graduate degree in a specialized area such as Computer Science or Forensic Accounting. According to them degrees in Law were no longer in high demand. There are also fields that require very specific degrees such as Forensic Medicine (which requires an MD), Engineering, Chemistry, Cryptology (which requires heavy duty Math) and Languages (Arabic, Hindi, Mandarin, Persian, Russian as well as French and Spanish). My Capstone adviser was employed by the DoJ Office of the Inspector General and was responsible for doing field observations, inspections and audits of all federal LE operations in So. California, Nevada and the four corner states. he dealt with the Border Patrol, INS, DEA, FBI, US Marshals, IRS, USDA, even the Postal Inspectors. He started as a local sheriff's deputy in Tennessee, got his degree on the GI Bill after Vietnam and joined the ATF hunting moonshiners and pot farmers up and down the Appalachians. He later got his MBA from Georgetown and got another Masters in accounting and was one of the first forensic accountants in the ATF (and all of federal LE outside of the IRS). Still works as a consultant to various agencies since his retirement.

I recommend to people thinking of getting into law enforcement to avoid a criminal justice degree. That may seem counter-intuitive, but the reason for this is that a criminal justice degree is not required to get a job in law enforcement and it is not easily applied to other fields. Law enforcement is a demanding field and a huge number of people find that it is not for them after a few years. A better plan is to get a business or computer related degree. You should also minor, or at least take a heavy course load in a foreign language. Criminal justice degrees rarely require foreign language courses, and although this makes them attractive to many students, it does them a disservice. There really is no better resume enhancer than fluency is a foreign language when it comes to getting a Federal 3 letter job. There are also great international jobs that are available after a career in LE if you have language skills, such as with OSCE or other agencies. Language skills will determine your assignment as well, so if you like the Northeast, French may be a better choice than Spanish. Similarly, Spanish fluency will likely draw you south or to major metropolitan areas. Other languages in high demand are Russian, and the various languages of Asia.
The application process for Federal LE is long - 6 months to a year is fairly common. The thing to keep in mind about Federal LE is that it is much more like the military in terms of where you will be located. You will almost always be assigned to a large city for your first tour in the FBI, USSS, etc. Field office assignments in nice places are hard to get. If you can get on with a good size police or sheriff's office before going to the feds, that can be really helpful, but NEVER tell anyone that is your plan. (State and local agencies lose some of their best people to the feds and they don't like investing in people if they know they are going to jump ship.)
Don't hesitate to shoot me a message if you would like any specific advice. I have worked extensively with many Federal agencies and would be happy to provide any insight I might have.

I work with placing veterans in jobs, my contacts in the FBI and DEA, all say a degree is justice helps, but anything in a business field, like ethics (works in the fraud department), chemistry (drugs), IT (cyber security). I would recommend that you got to ONET.org

The best degree would always be English and political science when they had a job fair they told me the degrees they like.

Off the top of my head and looking back on working with the Alphabet Agencies:
FBI=Accounting, Business, Psychology, Computer Science, or a Law Degree.
DEA=Criminal Justice, Chemistry, Sociology, or Psychology.
CIA= Criminal Justice, International Relations, or Psychology.
Of course any investigative or foreign experience is good. They have given more weight to those who have served in the Armed Forces and those who have advanced degrees.

Depends upon which alphabet group you are looking at? Some will like Juris Doctor, some will not require a degree? Language training is always a plus...but like the general question, a lot depends on your specifics.

I recommend a degree in Criminal Justice with emphasis in psychology. To work in the federal system, I would also recommend a Masters. I know former fellow officers who have gone this route and are now employed in federal law enforcement. The CIA is not an enforcement arm and thus, CJ would not be a good degree here. Focus on languages or international studies if this is your area of interest.

I have regularly, for several years, looked through USAJOBS at various jobs I thought I might be interested in. It doesn't matter which agency you want; what matters is what job you want. The basics for IT are the same throughout; the specific functions they may want you to perform can be learned there. The FBI is the most specific. For street agents, I am not sure the degree matters. If you want to do accounting, do accounting. If you want to be an attorney for the FBI, go to law school. It boils down to: what do you want to do?