Of course this is going to come down to personal preference, but the trend for me in terms of scenarios that get replayed the most I've found that it is the 2nd scenario.

Core set: Journey Along the Anduin

Kahazad Dum: Seventh Level, although other players might choose Into the Pit

HoN: Into Ithillien

VoI: To Catch an Orc

Black riders: Knife in the dark

Hobbit saga for me does not follow this trend, as well as AP releases, but for the most part the 2nd scenarios have seemed to deliver the quests with the most amount of replayability, offer a good challenge, and are the ones I like to test new deck builds against.

Any trend here for other players, or am I the only one?

For me its:

Core: Passage Through Mirkwood

HoN: Peril in Pelargir

VoI: Fords of Isen

Black Riders: Knife in the Dark.

Hmmmmm for me it is:
Core set: Journey Along the Anduin
Kahazad Dum: Seventh Level
HoN: Peril in Pelargir/Siege of Cair Andros (awesome quest but very hard)
VoI: To Catch an Orc
Black riders: Knife in the dark

So I'm the same and agree minus HoN

In some cases I think the second scenario tends to be the more balanced one in terms of difficulty, although it's the harder one in the case of HoN and VoI.

But my favorites are the same as Tracker1s. Those are all scenarios I keep coming back to.

Mine would be:

Core: Journey Along the Anduin

Khazad: Flight from Moria

Heirs: Siege of Cair Andros

Voice: To Catch an Orc

Black Riders: A Knife in the Dark

With Tracker…..

Same as Tracker also....although Flight From Moria is really cool with 3+ players....

It's not surprising that the second quest is usually better than the first. The question is though: why is the third quest not better than the second? In the core set the reason seems to be that the third quest was almost unbeatable solo and in multiplayer a tough nut with only the cards available in the core set. The other expansions didn't have that problem, but it appears to me that the designers tried to put too many things in these final quests to make them outstanding. Not that these were bad ideas, but IMO the third quests are most of the time too complicated which is why they're often versatile - either too hard or too easy. Sometimes they include badass enemies, and if you don't draw them at all it's a walk in the park. If you draw them too early, you're toast. Second quests seem to me more straightforward and thus easier to balance.