So far, this analysis seems to mostly assume that you're using haste to attack. You can also use the spell to Dash, Disengage, Hide, or use an object! Bearing this in mind, the bard might actually be a good pick!

Bards can easily be proficient in stealth, and their expertise feature lets them rival rogues for sneakiness. If your bard is like this and will be sporting a +8 or +9 stealth modifier (or ever a +5 or +6 from ordinary proficiency) at level 5, granting the haste for that hide action could be a very good move. And, of course, if your bard doesn't happen to be in a good position to hide, they could always attack. At these levels, a bard's attack tends to not be much worse than a fighter's attack. You can still be dealing an average of 6 or 7 damage.

Cast haste, target our biggest damage dealers. You become main target and often will lose concentration causing both your biggest damage dealers to lose a turn you use 3 sorcery points and a 3rd lvl spell to handicap your party.

Got a rouge or a shape shifted druid? They can make great use of it. Then all the other non spellcasters physical attackers. 

Whoever needs it most based on your groups PlayStyle.

Barbarian can deal a huge amount of Pain with a single attack. 

Cleric with +2 AC in full armour and dodging each round…

Bard casting Spell and useing the bonus action to Countercharm can be situational nice.

Fighter if he got a big damage option like GWM or SShoter.

I actually see the smallest benefit with the monk, an extra 1d8+4 won’t make a huge difference and the AC is low to begin with. He can already burn through all his Ki in two rounds anyway. The possibility to disengage is nice though.

A lot of who you SHOULD cast in on, for best effect, is going to be determined by group makeup and situation.  In a big fight, you can choose between your bigger damage dealers, or your skirmishers (Monk, Rogue, etc)  Helping the damage dealers is a pretty obvious benefit, while helping your skirmishers will better mitigate damage, as the extra AC might allow them to STAY front and center, offering an alternate target for enemy melee attacks.

Haste on almost any caster is close to a wasted spell slot, as casters don't get as much benefit in combat as the martials do.  In exploration challenges, or chases, it might help to give a caster the boost, allowing them to stay close (chases) or quickly get through a difficult spot. 

In all, the spell can boost damage output for anyone using the Attack action each round, add a little AC for those facing the brunt of the melee attacks and add mobility to those who may want to make sure they are NOT in the middle of a big scrum.  This means there isn't really a "right answer" to the question, as there are too many variables for any one reply to be "right"

Well barb is a helicopter of doom with haste

monk is a stun lock machine with haste but

everyone forgets the Bard give him haste and he can play misserlou at double speed on his Lute until it summons Nicolas Cage who then high fives the bard with a mighty thunderous clap

Or not