Well, if there was an easy answer to such question, everyone would be playing just that format, don't you think?
It highly depends on what do you like and what you are capable of as each format differs in several key aspects.
As a sit & go player you will need to master mostly push/fold play and play a ridiculously huge number of tourneys, which many find exhausting and especially boring. You will have to be able to play 1-2 hours sessions, sometimes more of them in one day and, most importantly, handle short term swings, as +/- 10-20 buy-ins in one session is quite a standard. You will have to deal with tons of bad beats and setups and you may run under EV for eternity.
MTTs are without the doubt the most fun when in comes to tournament poker, but also the hardest in my opinion as your sessions may and will last 8+ hours and the possible swings are incredible. You will never have an idea at beginning of the month, whether you will end up being +400BI or -250BI and because the pay outs are SOOO top-heavy, just a few lost coin flips on final tables may cost you most of your potential profit in a certain month.
Cash games, on the other hand are most flexible, but also toughest to master. If there are plenty of fish and tables on your limits, you can basically play anytime you want to for any long you want to and take a break whenever you want. However, you have to be 100% focused every single hand you play as even the "tinyiest" piece of information or read can make an otherwise losing play incredibly profitable against a certain player.
This is just a very short summary of pros and cons of different formats, but I am pretty sure that you get the point I am trying to make.
