Class

[armandojc3]

Ninja hands down. Complex, unpopular and weak.

[Taranok]

The who's who of unpopular jobs right now are:

Summoner (Bad rotation, lowish damage, poor flow, overly complex, too much to keep track of)
Ninja (Bad flow, low perceived damage, poor design of trick attack ruining how other abilities interact, possibly too much to keep track of and fire in too short a window)
Machinist (Bad tolerance of ping, slightly janky rotation, damage a bit undertuned) though Machinist is slowly gaining in popularity as the only major problem it has is ping, which is unfortunately a love-it-or-leave-it situation.
Bard (Minor-ish flow issues, has a bit of an identity crisis now because it's no longer the support rDPS). This is mostly the identity crisis, not rotation or complexity problems.
Monk (Smoothed out and streamlined rotation, but a lot of abilities that overlap or compete with each other instead of complimenting each other, core rotation hasn't changed since ARR except to get slightly simpler from the removal of an at-will DoT, and stale design have pushed people away).

Most other jobs are at least not unpopular for any reason or don't have a major, glaring issue.

Summoner and Ninja have it the worst by far, with massive flow problems, oGCD issues, and very little payoff for good execution of its complexity. SMN has arguably more issues in that it has uncontrollable DPS loss from forced movement while demi pets are out, but if you ignore that issue then they both appear to be relatively equal in badness right now.

The flow issues are the biggest problem. Summoner flat out doesn't flow, so despite having fewer APM than Bard or Machinist, it feels infinitely worse and more complex because it doesn't flow and little else.