These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
First Installment Wins: Both games are almost on par, the first one benefits from a more creepy, cavernoid and less humanoid atmosphere and the second one has several improvements. What tips the scales in favor of the original is the KeeperFX Fan Mod, as it keeps the game very alive and fresh three lustrums after its release; the community is fairly active and new scenarios and updates appear on a regular basis, while the sequel doesn't receive much love. And let's not even talk about the mobile version...
Game Breaker: Convert the Lord Of The Land to your side... Then use the special items to carry him to the next level. You would be amazed how easier the next levels get when the initial waves of heroes are face to face with a stage boss that works for you. In the original, 16 of the 20 levels have an item that lets you carry a creature over to the next level and unlike Horny, the lord of the land is much less fickle, and almost as strong.
The Dark Knight is absurdly strong for his cost and is easy to amass in large numbers. It got to the point that he's limited (if not outright banned) in multiplayer. Compare to the Dark Angel, who is even stronger but is considered balanced due to his ludicrous cost.
Most Wonderful Sound: "JACKPOT WINNER!" You will drop anything, ANYTHING you are doing, up to and including micromanaging the destruction of a rival Keeper's heart, to zoom over to the Casino and watch your various creatures dance to Disco Inferno.
The Imps can be this as well, on some maps. They'll claim enemy and neutral territory whenever they can get to it. This can be a problem, especially if you're trying to avoid scripted sequences or butting up against an enemy's territory. Sure, you can lock them in a special room, but then you remember you need a few imps to mine gold and move traps.
That One Level: "Woodsong" in the second game. You're on a time limit and have to build up a massive force, rush your way through an enemy encampment and lay siege to an enemy keeper while the whole heroic army bears down on your army (and your now-garrisoned dungeon) from behind. And if one of the heroes deals the killing blow to your enemy, you fail the mission. The 1.7 patch makes the level much easier by quadrupling the time limit. You probably still won't beat it, but the reason is completely different from before.