Just played the "classic" trilogy, reviewed
WT and
SM.
Tend to agree that the first one is the best in terms of the overall "feel", but the sequels definitely improved on the gameplay. Not having the "wizard", so the heroes have to cast, severely limited magic in the first, for one. And there's also the lack of a combat turn limit, which can force you to lose fights and units because otherwise the game softlocks basically (you're left with unit(s) that can't defeat the enemy but are too fast to be caught and killed, in later games you can run around until the turn limit and get a draw, in the first you need to lose to end combat).
WT has the weakest story but the strongest heroes, can be played more like an RPG, some are bothered by that, I definitely am not. It also has a very exploitable AI, and no ship AI, so have a body of water between you and the enemy and you're safe, and there's a spell that slowly floods the map, so go figure. But great for defensive minded players, with some good magic and fortifications and a lot of care the large majority of the time a town can be defended with two neutral monks, admittedly in good part also thanks to the laughable siege AI. Can be hard even so, and at the end it definitely is, but still struck me as fair.
SM brings a bunch of changes, good and bad. Some more story and even atmosphere, but tries too hard to be funny and that doesn't fit. Somewhat better AI, some new mechanics and features... But much weaker heroes, added limitations to units that were OP before (again, many would probably like these, but for playing the campaign against the computer, not so good). But my main problem with it was the automatic surrender. If an army faces overwhelming odds, it'll automatically surrender, even when defending a city and even when attacking (!), so you can attack an enemy and your units will surrender. So no way to defend cities with a couple of units anymore, no way to grind for exp on weak enemies, no way to soften enemies with weaker attacks, or hold them back with draws obtained with weak but fast units that can keep running away...
In the end, depends what you prefer. Pros and cons to each.