Based on what we've seen in Daggerfall, Morrowind, Oblivion and Skyrim which guild of cutthroats do you think is the best?

DB all the way.

Well, the one time the Brotherhood and Tong crossed paths, the Brotherhood got wiped out.

Both have assassinated Imperial Emperors, and both are mere shadows of their former glories. Overall, I believe the Brotherhood has been more influential in Tamriel's history. After the Morag Tong assassinated one of the Reman Emperors and a few other high-ranking nobles, they were hunted to extinction outside of Morrowind. The Brotherhood retained a significant in all provinces for a long time.

In terms of power, I go for the Brotherhood due to sheer numbers. Coolness, I go for the Brotherhood simply because we've seen more of them (which is a terrible way to rank it, but there it is).

All Morag Tong, the dark brotherhood will always end up killing itself or being killed off only to be rebuilt as a weaker force as seen in previous games.

The Dark Brotherhood seems to have fail a lot too when tasked with successfully penetrating the Imperial prison or assassinating Adamus Phillda up until the COC got involved but at least they were badass and cool in Oblivion. The ones in Skyrim are a load of Guar turd in comparison. I'm a little fuzzy on the Morag Tong's success rate but I think they're probably better at fulfilling their contracts though that could be because of their legality inside of Morrowind.

At the peak of the Morag Tong's power, I believe they had presence all over Tamriel. During the Reman Empire, they assassinated Reman I, Reman III, Prince Juliek, Potentate Versidue-Shaie, and Potentate Savirien-Chorak along with all his heirs.

Yes I already know about those those infamous assassinations, I just wanted to know about the failure rate in the Morag Tong(if there is any at all) and how it compares to the Brotherhood. Personally I prefer the Morag Tong because their purpose and methods are more akin to my liking.

True.

Pffttt the DB only took out milkdrinker Emperors unless you count the PC doing it for them which shouldn't count. Morag Tong took out the Remans and ended their Empire and then turned around and ended the Potentates as well several hundred years later. Those two events alone are more influential than anything the DB has managed to do in all their time as an organization. Relatively speaking the MT may have maintained a significant presence across Tamriel for a longer period of time than the DB, if we include the first and second eras, and they still operate outside of Morrowind under the Grey Writs. At the time of Skyrim the DB seems to have almost hit as hard of times as the MT. I would say collectively the DB is a more powerful organization but individually the MT are more powerful.

Okay, I do not see how the DB wins the coolness vote by even any kind of margin. Even if we have seen more of them we did get to be part of the MT in Morrowind and they were much cooler in that setting than any of the DB members were in other games. Plus as an organization they act as a form of peace keeping that keeps/kept the Great Houses of Morrowind from outright warring with one another. The DB are cut and paste religious fanatic assassins. I understand this could be a case of personal opinion but I actually think the MT are almost objectively cooler than the DB. But meh...