My own take on this at the moment, in a mutator free environment.
T4
Queen: A+
Queen has screech, explosive resistance, fire immunity, and pheromones. Those are just combat abilities. Utility wise the tac map, overwatch on any xeno, assigning leaders and making resin structures makes her the most versatile unit in the game. Her only disadvantage is speed.
T3
Prea: B
Boiler: A
Crusher: B
Rav: C+
Boiler gets an A due to wider gas ruining pushes and FOBs. Prea and Crusher get B due to high versatility and reliability respectively. Rav is close to B, thanks to fire immunity but falls short thanks to low health and armour.
T2
Lurker: C
Warrior: B
Spitter: C
Hivelord: C+
Burrower: D
Carrier: C
Warrior with lunge, grab, and a good balance between damage and health/armour/agility wins out T2. Lurker, Spitter, Carrier and Hivelord are average. Burrower falls behind for a lack of versatility.
T1
Drone: C
Runner: D
Sent: C
Defender: D
Drone and Sent standout at T1 for high versatility and highly reliable stun respectively. Runner is too highly focused whilst Defender's lack of mobility holds them back. Honestly Defender probably deserves a higher mark but I don't want to give every caste a C or B.
Others:
Larva: E
Predlarva: terriblesprite/10
Predalien: A+
Xenoborg: eventplz/10
Total distribution:-
A: 2
B: 3
C: 7
D: 3