1. Paladin - I really like how he deals two powerful blows with his huge sword.
2. Titan - the one that throws lightning and can also hit hard.
3. Golems - because of their boxing ability and the fact that there are four types of them.
4. Mermaid - simply the first aquatic unit in the series. Of course, there are also pirates and a sea monster, but they rarely appeared.
5. Ballista - especially when it's fast-firing and with fire arrows. Among those, the Imps with their bottles, which steal mana, are also cool.
6. Definitely Kappas - I really liked how they jump and sweep across the entire map with their legs. These are also cool invisible units from the Dungeon faction.
7. I haven't played much, so I don't remember many units. I'll write more later.