It all depends on what you’re interested in. Melee? Ranged? Magic-user? One keypress or several to do the same job? Busy and concentration/timing dependant or blindly button-mashy? PvE or PvP?
If you want to win out of the box in PvE and have good reflexes, and you like straightforward, powerful-feeling melee gameplay, Warrior is the one to go for. If you want to win out of the box but your reflexes are shot from years of self-abuse, and you also like your gameplay a bit more fiddly, Guardian is the one to go for. Those two are the safest bets, for sure.
If you want to be feared in PvP, the current meta is Necromancer and Engineer (both of which are quite fiddly classes, so again, if you like fiddly, they’re great). If you love pet classes, Ranger is a great pet class. If you like stealth-based gameplay, Thief obviously (and it’s well implemented). Mesmer and Elementalist are comparatively more unforgiving in the early stages - but to compensate, they have fun, spectacular fx and are very powerful and loved in teams, later in the game. And of course as befitting magic users, they have uber fiddly, busy playstyles, highly dependent on good timing and familiarity with the dodge mechanic for their survivability.
Generally speaking, PvE is forgiving enough that you can blindly button mash for quite a while and do ok - then eventually some sense of what you’re actually doing will emerge from the mental fog (read the tooltips!), and you’ll discover some headroom and depth, to get more efficient in.
I went straight for Mesmer and never regretted it (I also enjoy Warrior, Guardian and Ranger for occasional breaks). Yes it was a bit tough up to about lvl 40 or so, but it did teach me to dodge and use the Mesmer’s very active form of defence. Plus, the fx are superb, and just scream “magic-user”; splashy and big, which is extremely gratifying (nobody is ever in any doubt that a Mesmer is on the scene!) Plus, Mesmer can be the trolliest of trolly classes in PvP, even trollier than a Thief - always a big plus :)