I haven't played much of HOMM5, but I've played a lot of both HOMM2 and HOMM3. HOMM3 has way more "stuff" and is more balanced. There's more monsters, maps, and castle types (spells are almost the same). However, HOMM2 is both mechanically and visually more impressive. I'm usually not into graphics much, but pixel art of HOMM2 looks so good. Even HOMM3's pretty map assets are mostly just taken from HOMM2. Mechanically, HOMM2 has a lot more unique creatures and castles due to massive strengths and weaknesses. Flyers, for example, can move anywhere on the map, totally disabling ranged units. However, ranged units are still some of the most valuable due to their ability to mow down pretty much every other type of unit without recieving counter attacks and therefore not taking any losses. My favorite unit in the game is the hydra, a big hunk of flesh that doesn't get counterattacked and hits ALL neighboring enemy creatures when attacking. On the flip side, it moves at a snail's pace and is totally useless against ranged units if you don't have haste. Every single creature has its own identity and importance (except mummies). Castles are also very tilted towards a particular playstyle, rather than having all the fun balanced out. Barbarians and Knights can get to their good stuff really fast, not needing all those extra non-wood/ore resources to get there. HOMM3 castles, on the other hand, don't have any major strengths or weaknesses.
