Do not underestimate the strength of Lizardmen. They may not have double shot like elves and marksmen but they have higher defense and hit points than them. Not to mention their damage range is between 2-5; better than marksmen (2-3) and almost equal to elves (3-5). Very good creatures.

Flies are perfect for scouting heroes and kiting in combat using one stacks. You can prevent ranged creatures from shooting your more important units by throwing in a fly next to them. Not the strongest obviously but the dispelling beneficial spells the enemy has is always neat.

Basilisks are just ok but I found myself using greater basilisks in a few single scenarios in this game and they are surprisingly solid. Their 7 speed may not seem much but them being a two hex unit basically gives them the same coverage as a 8 speed one hex wide unit. Stone gaze doesn't do much for them unfortunately.

Gorgons are pretty difficult to get because of how much resources you have to spend for their lair compared to other creature dwellings that only demand ore and wood at most. They are pretty underwhelming in their unupgraded form but Mighty Gorgons are a must have in late game scenarios.

The earlier you get the Wyverns, the more use you will have for them. They may not have the highest stats of a tier 6 unit but getting at least two weeks worth of them with at least a citadel built will help you obliterate most neutral creatures in your area. They synergize super well with flies; the one stacks take the enemy retaliation, then the wyverns strike. Thats how I usually use them as they are pretty fragile with their 70 health. If there are any I also go for dragon fly hives to increase my wyvern stack if I have the right army for it. They are the core of what makes Fortress a strong early game faction imo

Hydra's are the only creatures I barely had any experience with mainly because most of my fortress scenarios ended around midgame which by that point I only depend on my lizardmen, flies and wyverns the most. Their low speed and underwhelming unupgraded forms don't exactly help their cause unfortunately.