I heard much about original campaigns of HoMM2. For now, I got through Roland's chapter. Stuck on a final scenario and almost hate this game. I going to give the last chance and try one of Price of Loyalty campaign (there are four of them).
So which one I should try?
Roland's campaign doesn't have much of story and it upset me.

What about it frustrated you? Did you choose troops or the Ultimate Crown? Even with the troops, the AI can be pretty annoying in that final mission.

As for the expansion campaigns, each one has their own unique challenges. Voyage Home starts out really fun, but the last mission is one giant headache. Maybe play the first two missions and then stop. The last few missions of Price of Loyalty might be tricky due to the AI having massive armies. That isn't to say Descendants or Wizard's Isle are easy, but I think they're less frustrating than the other two. I'd start with them first.

I chose troops but I think I don't get enough of it. I used different tactics and in results of my best attempt, I seized two castles on the east and one on the west. After a few attacks, the enemy comes with a bunch of heroes and some of them have 5-10 dragons of different types. Yeah, you right. It's greatly annoying. I have not any chances to evolve my castles and gather a good army. Thank you for the answer. I'll try it.

From the troop-gathering mission you could have brought 19 black dragons, well over three hundred battle dwarves, plus thirty or so arch mages, forty unicorns and sixty greater druids.

Roland's last map (the showdown with Archibald) has a twist, which when you know it can be exploited in subsequent plays.

Objective of the campaign is to complete your chosen missions in the lowest overall total of days - 300 being the target ("black dragon" score).