I is probably my favorite, it’s definitely I or II. You’re certainly in the minority thinking it’s the weakest of the three. I remember when Jeff Vogel released the remake of the second one he remarked that it had always been the most popular game in the series. I had friends who were Exile fans of back in the day and they also thought II was the best one.

III is my least favorite of the original trilogy because I do think the giant map with identical towns gets wearying. I didn’t finish the golem quest in Avernum III, but I did it in Exile III. I’ll do it when I replay Avernum III.

My rankings from least favorite to favorite:

II probably > I, but this is the toughest call
VI
III
V
IV

but don’t let my low rating for IV make you less excited to play it. I’ve only played the old version and the new one is being warmly received. I do think that IV has the weakest story. On the plus side, it has a clearly defined end game that I think you can do without beating any of the other main quests. I think there are 3? At least 3? I didn’t finish one, but didn’t even understand that it was unfinished until I was reading about the game later, because that storyline doesn’t get truly resolved until V anyway so from an story standpoint it doesn’t feel like it mattered.
Anyway, I thought that VI and the remakes of I-III had improved gameplay and better graphics (the original IV often looked like a big cement basement) so I expect to like these versions of IV and V more. And I did still like the original IV and V.

V is the most linear game, but I remember there being points where you can only do one quest or another, so maybe it’s the most replayable. I also expect one big complaint about that game to be fixed: there are alot of boss-type fights, and Jeff Vogel asked the playtesters how they did them and then tried to eliminate any “cheap” ways of beating the bosses, which ended up with alot of tough, sloggy battles. This was a common complaint, so I expect it to be improved.