2 questions

The Assassin Rogue is good at alpha-strikes, which means it's all-or-nothing.  If it pulls the gimmick off, the boss battle likely ends anticlimactically from the monster-load of damage hitting the boss before it even got to do anything.  If the attack misses, or the boss has significantly more hit points than the damage dealt... the Assassin Rogue is now just a Rogue for the rest of the fight.  In my opinion, the Swashbuckler Rogue is significantly better, because it's more versatile, able to more reliably get Sneak Attack damage, and it's less likely to deny the rest of the party the big boss fight they've been working towards.

Wizards are good, but Druids give them a run for their money, especially L20 Spores Druids that are able to replenish 80 temporary hit points every turn as an action, ad-infinitum, and can combine that with the L7 Draconic Transformation to use a Bonus Action every turn to exhale a cone of Force Damage.

A Wizard may generally be better than a Sorcerer, but in a head-to-head, the Sorcerer could Subtle Counterspell everything the Wizard tries, and Subtle cast his own spells so the Wizard can't Counterspell them.

Wizard is the best class in the sense that is has the most useful options at all tiers of play. But it is one of the worst classes in terms of actual class design, since it derives all of that usefulness from the superior spell list it has. The subclasses of wizard are neat but don't even change your primary style of play all that much.

A level 5 evoker and a level 5 necromancer and a level 5 illusionist and a level 5 abjurer and a leve...etc, all have the same solution to a ton of weak enemies or a whole swarm of stronger ones. Either Fireball or Hypnotic Pattern. They have the same solutions to most problems. because they all have the same spell list. And most of their usefulness comes from that list.

The only subclass that gets close to breaking out of the mold is the bladesinger. And only at low levels. At high levels you're just a wizard with a high AC. But many wizards figure out ways to get high AC anyway so it isn't even all that unique even then.

So yeah, amazing class because it has a solution to all sorts of everything. But class design is super vanilla plain because all the cool stuff comes from the spells which isn't even class feature or subclass feature stuff.

I would tend to rate bard above wizard, and druid can come in pretty high as well. It also depends on assumptions about how much time you spend in various tiers of play.

I love the Assassin. I used it in my friends adventure and did over 30 damage with one hit at level 4. I plan to take two in fighter so I can get two weapon fighting and action surge. I am loving the idea of doing 10d6+10 damage in turn. 

The wizard has an absurd spell list but no way to get a nice amount of hit points. I would not like having to write spells in your spellbook before using. 

Wizard is the best class because pointy hat and big staff and long beard

Necromancer wizards can make fortresses filled with traps and magen and have an army of skeletons equipped with the latest most fashionable equipment that they can wield to protect you and more. What could a fighter do about that?