Fang: Chaff (cannon fodder)
Crawler: Fast-moving chaff
Marksman: Sniper, base range is 140
Arclight: Ranged chaff clear (splash damage)
Hound: Chaff clear, squad of five, less range than Arclight

Sledgehammer: Squad of five with medium HP, has small splash
Steel Ball: Fast-moving squad of four with medium HP, laser attack (laser ramps up when it stays on one target)

For comparison, an Arclight has 4000 HP, a single Sledgehammer has 3500 HP, and a Steel Ball has 4500 HP. Marksman ATK is 2300.

Stormcaller: Artillery squad with rockets that can miss, and can be intercepted by missile interceptors
Sabertooth: Big damage (8000) with 15000 HP (has small splash, as do many other big-damage units, but its purpose is single-target damage)
Tarantula: Chaff clear with 15000 HP, less range than Arclight
Mustang: Fast-moving, ranged, low-HP "horse archers" dealing fine-grained damage

Mustang is weak at lv.1, but its fine-grained damage scales very well due to lack of overkill. Arclight and other chaff clear units are also somewhat capable of scaling well due to this phenomenon.

Rhino: Fast-moving, melee, 20000 HP
Hacker: Mind-control

Wasp: Flying swarm of twelve, can be chaff against snipers, deal decent damage if there's nothing to clear them, but easily targeted due to short range
Phoenix: Flying sniper squad of two, base range is 120
Phantom Ray: Flying squad of three with medium HP (3200 each), short-ranged 2*1000 torpedo attack with miniscule splash, several interesting techs

Wraith: Flying chaff clear with 15000 HP, short range
Scorpion: Artillery with 18000 HP (10000 ATK, big splash, doesn't miss (unless you tech Siege Mode then maybe), doesn't count as a missile so it can't be intercepted)

Melting Point: Giant with laser, base range is 115
Vulcan: Giant chaff clear
Fortress: Kinda like a giant Sabertooth, but with different techs, notably Anti-air barrage
Sandworm: Giant melee, hides underground when moving

Raiden: Flying giant with forked 3x5000 ATK (different targets only), base range is 110
Overlord: Flying giant artillery, with torpedoes that travel faster and scatter less than Stormcaller's, and therefore miss less

War Factory: Titan with four guns; basically four Fortresses stapled together. Use the Movement Beacon to force it forward so that its hind guns can actually hit things.
Abyss: Flying titan with a unique linear AoE attack. Newly released, feels chaotic and high-variance.

Fire Badger: 200-cost rare unit. Squad of three with medium HP (5000 each), medium speed. Short-ranged chaff clear.
Farseer: 300-cost rare unit. Well-rounded with good base stats. Fast-moving, so be careful about putting it too far forward. Long-ranged torpedo attack that can be intercepted.
Typhoon: 300-cost rare unit. Its fine-grained damage actually includes a tiny splash component. Has 100 range which is same as many other units, but no Range Enhancement tech!

Here are some ideas for where to place various units. (Not an example composition, just a bunch of example placements compacted into one image.) Chaff goes in waves. Other slow units by the towers. Other fast units in the back half of the board.