I play on very large maps (only random ones, as it's more interesting) against 7 AI opponents with one team at 200%. Sometimes I choose Dungeon, but more often I play without it.
Usually, by week 2-3, a couple of AI opponents come to visit.
As a result, with Swamp, you have to defend more than attack. Slow heroes, sluggish units (it's difficult to win without losses, as even with speed boosts, the AI can charge up Chain Lightning), no 4-5 schools of magic, and the warehouse doesn't provide sulfur.
For comparison:
Undead - unbeatable. Widomakin and the AI doesn't matter. By the end of the game, there are 10,000 skeletons wandering around.
Elementals - birds + fakes + storm riders, and again, the AI and monsters don't matter.
Dungeon - minotaurs + 2 archers + harpies without retaliation - that's cool. Especially with doubled mana, a decent school of magic, and good heroes (like Gunnar). And as a bonus, a cheat portal summon + magic immunity for the fighters = hello Armageddon, if it appears.
Humans - a couple of conservatories (and they almost always appear, unlike Hives) and you get a bunch of angels, which can handle things even without upgrades. I won't even mention what happens when they become archangels. + 2 archers. With Orin, they deal good damage - perfect for the beginning. I don't use diplomacy.
Magic - Chain Lightning from the start with the hero... a great school of magic... meaty units and a lot of archers. Protect the archers and let the AI hit golems and gargoyles head-on.
Elves - excellent archers for the start (later replaced by something else, as they don't live long), thick dendroids and gnomes, decent dragons, and fast centaurs. There's a great hero for the start - the Logistician.
Orcs - the lack of crystals is a problem. If there are crystals, you get monsters in the first week... And then a quick clearing of the map. Insane damage from the hero. The downside is that they are slow, like Swamp, but this is compensated by the damage + monsters.
Inferno... I don't like it. The troops are too weak. Only the Ifrits are decent.