Depends on what you mean by 'effective'. Fighting in a ring (tkd, boxing, BJJ, MMA, judo, etc) with rules, some random brawl in the train/streets, or fending off a robber?
Some years ago, a punk tried to rob me while I was on holiday. I put him down on the ground before he realized it using Aikido, a martial arts whose effectiveness on the streets or the ring had been debated intensely, often with great scepticism. I don't know how my martial arts would rank, but I just need it for self defence. Enough to keep me alive, and if possible, neutralize my opponent within seconds. I am not into ring fighting or going 10 rounds with my opponent.
So before you pick your training arts, you should decide what you want it for - keeping fit, ring fighting, self defence, a mixture those reasons. Ultimately, what you get depends a lot of how much effort you put into your training.