This is actually a pretty hard question to answer, because everyone has their opinion on what is the best DD job. I'll paste the job pages for the jobs I think are the best DD's below.
I've always thought that hands down that Monk was the best DD job if you are equipped properly - keep in mind all jobs will be "if you have equipped properly" - Monks have a high TP gain, hit hard and fast and you have 100 fists plus the newer weapon skills. Monk also has a very high HP pool that goes with the job, but the down side is that a Monk isn't made to tank - so you really don't want to grab long term hate. When choosing your H2H weapon, invest in a really GOOD set, low delay with a high damage rate is best.
Ninja would be the next DD I would say. You have all the different tools that you can use and Ninja's have a lot of built in job abilities that are pretty decent along with a high HP and TP build allowing them to get off some serious weapons skills. Again when choosing your Katanas, low delay and high damage rate - and you do not have to use the different magics. I've never been a magic using Ninja and did just fine, but I subbed Dancer to keep my butt healed.
Samurai is one that you really have to KNOW the job - and if you know it really well you can get massive TP build, self skill chains on mob and do outrageous amounts of damage with your weapon skills. They are pretty tough, they can't tank like a Paladin or a Warrior - don't except them to be able to tank - do except them to draw hate with weapon skills.....but should be able to withstand a beating from a mob for a short period.
In my opinion Dancer is an outstanding DD. It is one of my favorite. If you really know this job, you can take on a lot of different mob solo and with your Drain Samba and Healing Waltz you can do a long drawn out battle and most of the time be the last one standing. Here is the trick with Dancer, if you are in party, you will a lot of times find yourself being a frontline healer as you are slicing away at the mob. You will use your Drain Samba so that other can get HP off the mob and you will target other players with your Healing Waltz.....but if things go south, take care of your own health and ignore the other players. You will need every scrape of your own TP to keep yourself alive.
I know this from vast experience. I've been in many situation when it was coming down to a full party wipe and I focused on myself and I faced to faced kited out the mob, healing myself as I took it's health down with very fast TP build. The cool thing about Dancer is a job ability called Steps and Convert that allows you to build up to your full TP ready for a weapon skill in just 2 to 3 turns. So you are are sometimes doing weapons skills back to back. I took on Faust, which is a VERY hard HNM and managed to kill him has a Dancer because of Steps and Convert.
Thief is a controversial DD. This aren't really made to take a lot of damage, but they can really deal it out. With Job Abilities that are designed to make it difficult for the mobs to hit Thief, they do shed hate easily - also part of the job abilities is Triple Attack. If you get the right equipment to enhance this you can up your damage per second and be doing some major damage simply become you are striking your target much faster and more often than other player. I've been told by my husband when he watches my Thief on screen that it looks like I'm doing non-stop attacks....which he just said he noticed about Dancer too.
Sorry I couldn't give you just ONE job - but really there isn't just one job that is the best...you really need to try out different jobs to see which ones that you like. I love playing Dancer, Monk and Thief, but I also love White Mage, BeastMaster and Dragoon (which can also be a decent DD). I think the above cover the best DDs you can try out and see which route you want to go....and just remember - gear yourself properly so that you can do your job correctly. Skyfiredancer (talk) 11:54, October 6, 2015 (UTC)