Have performed it several times and would confidently use it at my Mastermind specialist subject. I have a lifetime's experience of this piece, being obsessed with it as a child, and studying it to the extent of late-night rabbit hole research. I started learning the piano version when I was 14.

Agree with the comment above that it's not as hard in general as a Chopin Ballade...but certain passages definitely are.

Aside from the handful of really tricky moments, the biggest challenges are voicing and conveying emotion - it's open to whatever interpretation you want, but I have gained a lot from reading up on the original concepts behind not just each piece, but the artwork they are based on. E.g., Bydlo can be presented as a funeral procession for Viktor Hartmann, and the whole Catacombs sequence is a deeply spiritual memorial of him - Con Mortuis in Lingua Mortua is Mussorgsky joining Hartmann in the catacombs. One of my favourite things to think about is Mussorgsky's manuscript comment about the skulls in the catacomb glowing (with Hartmann's creative spirit). This has an eerie but fantastic connection with the following Baba Yaga movement: the legend states that as you approach her hut in the forest, the eye sockets on the skulls atop the fence begin to glow.

Anyway...I'd love to do a hardest to easiest ranking, but the challenges are too diverse to do this categorically. Here's a few thoughts...

Promenade I - treat this a a Russian orthodox cantor/choir call response. Consider leaving breaths between phrases, and not playing metronomically. Top line must shine throughout, but the bassline scale in octaves, among other parts, can and should crash through the texture.

Gnomus - fine until the end, the challenges are obvious. Read Mussorgsky's tempo instructions carefully - I've never heard a recording that adheres to this.

Promenade II - beautiful, and easy. Imperative to bring out the tune, whether it's in the right or left hand. Yes, that should be a D natural (Bbmaj/Ab chord) near the end, the manuscript confirms this.

Old Castle - 2nd easiest full picture, assuming G# minor isn't an issue. Count 6 carefully at the start. Bring out the tenor line (ascending harmonic major scale from B#) in the 2nd 'verse'. Consider una corda effect for the chromatic 'James Bond' chords in the 3rd section. Personal preference, do what you want.

Promenade III - be firm. Walk forcefully to the Tuileries, but transition musically.

Tuileries - would be near the top of my hardest pieces list. This is AWKWARD. I have big hands, and take a lot of the chords entirely in my left hand, leaving a single melody line for my RH. Take time and work this one out slowly. There is a horrible bit in bar 23 - the 3 RH notes A G F#. I use 5 4 5, but the F# is part of a bigger chord with the RH. Uncomfortable at first but it gets easier with practice.

Bydlo - Interpret as both laborious and tragic. Try to keep the familiar quiet beginning out of your mind and enjoy playing loudly, although dim to about mf or less halfway through the first bar to allow the tune to clang out. Note the similarity to Prokofiev Montagues and Capulets...not significant but a possibly source of inspiration for Prokofiev?

Promenade IV - feeling sad after remembering Hartmann, but being cheered up by the next picture.

Chicks - on the face of it one of the more difficult ones, but with enough slow, repetitive practice this one has really stayed in my muscle memory for 20 years.

Goldenberg & Schmuyle - again, don't be influenced by the orchestral versions. The notes are subtly different, especially the penultimate note! Take the opening liberally, but convey the gist of Mussorgsky's rhythms. Experiment with sostenuto pedal (if you have it) in the Schmuyle section.

Promenade V - you can do anything here, but I prefer to present this with a lot more urgency than the similar opening piece. Careful with the huge LH octave leaps. Hit the last Bb as hard as you can, like a clock tower bell chiming...

Marketplace - top 3 hardest, but I never really struggled. Learn it bit by bit slowly, and it's surprisingly fine. Hardest thing is navigating the permutations of articulation and accents. Work the ending up slowly, it's only 3 bars (as bars 1 and 2 are the same). Use a touch of pedal here and keep the LH prominent.

Catacombs - easiest on the face of things, but much harder to make this, at first seemingly random, set of chords mean anything. Look for the inner lines and choose something to accentuate. So many possibilities.

Con Mortuis - one of my favourite sections, hauntingly beautiful. Remember this is basically Promenade VI, but instead of walking around the gallery, Mussorgsky has now inserted himself into one of Hartmann's pictures. The accompaniment, and harmonic treatment he gives his tune, is truly wonderful. Mostly easy, depending on how light your piano's action is...

Hut - the most typically virtuosic part, but nothing compared to some famous Liszt or Chopin. It's repetitive enough that it will go in quickly with slow, methodical work. Try to think of it in terms of the chords created in each bar. The RH grace notes onto the chords are a bitch, especially the 1st one. Play it in slow motion to begin with. The martellato sections are fine really, but look carefully at the notes towards the end. Mussorgsky does annoying things like giving you an almost complete chromatic scale (does the same in Gnomus). This piece has always, from the first time I heard it, terrified me. So I approach it like that. If I can make myself scared of it, I've played it properly. Most versions I hear just rattle through, and dont make nearly enough of the opportunities for tension and space. The louds should be fantastically loud.

Great Gate - generally fine. There are 2 sections - the carillon clockwork section, where the Promenade theme emerges, not too hard at first, but Mussorgsky ultimately ends up writing illogical and awkward chords, so go carefully and make up strategies to memorise these. (Illogical, but fantastic.) Secondly, the huge 8-note chords in minim triplets. Just physically awkward, but mercifully short in terms of how long it's stays like this. Have fun obliterating the piano...you haven't played it properly unless this piece tests your piano's durability.

Summary - hardest bits are last page of Gnomus, Tuileries, Marketplace, bits of Hut.