So, do you mean a slow movement or the second movement of a sonata? That is not the same, mind you, especially in Beethoven. Considering that you mean slow, here are my thoughts:
Let's begin with the end. All movements of the last four sonatas fall out, because they are
a) extremely long (we are talking op.111/2, op.110/3, op.109/3, op.106/3)
b) very hard
c) only understandable in the context of the sonata and should not be played as single pieces.
One exception may be found considering the late sonatas, and that is the first mov. of op.110. This is a moderato molto espressivo, so you could say it is a slow one, it is neither long nor very hard, and you could pick it, if you like the sound of late Beethoven. However, you probably will have the problem that you will urgently want to play the whole sonata, so beware...
Op.101 The slow movement in the orignal sense is the 3rd mov. But it is fairly short and is more of a introduction to the finale, it is quite easy as well. I would not pick it, it is too much connected with the 4th mov.
First movement however is quite slow (well, Allegretto ma non troppo), and is more of a stand alone piece (but not really! Btw, I find it quite a horrible idea to play only one Beet. movement, play a whole sonata. I am just replying because I am such a nice guy
). Even shorter and easier (technically) than the op.110.
Op.90 second movement. Way too long for your purpose I deem, and quite difficult. Makes the major part of the sonata, should not be played alone (very comparable to the op.111, Minor-Major contrast between the only two movements, many more parallels).
Op.81a Third movement. Bad idea as well, quite connected with the Vivacissimamente.
Op.79 A sonatina, very easy and short second movement, but if you are playing concerti already, it would be nothing more than a sight-reading exercise.
Op.78 Does not have a slow movement-
Op.57 Second movement -yeah! Is a pretty stand alone piece, nice set of variations, quite challenging (especially the third variation with the 32th...). So beautiful, really, you think there is this marvellous melody, but then you realize, ther is none! Fascinating...
You would have to compose an own ending though , because the last variation introduces to the final as well...
Op.54 Two movement sonata, one Tempo d'un Menuetto, one Allegretto. Neither is really slow... I think one of Ludwig's worst sonatas...
Op.53 Introduzione (2nd mov) is slow, but no real piece to play if you don't play the third movement. It is what it is. An introduction.
So you see, from the last 12 there are only some options: op.110/1, op.101/1, op.57/2.
Op. 49. Too easy
Op.31/1 second movement: Very "solistic" as if it was a concert, you know trills, fast runs, arpeggios all over the place. Quite demanding, not really a lyrical slow mov. if you are looking for that. Could help you however with certain techniques (I think the bass accompainement gave me some hard time...have a look)
Op.31/2 Second movement. Personally I rather dislike it...However, it is very slow and has some nice runs in it...Not too hard though, worth a try if you like it.
Op. 31/3 Quite hard to determine the slow mov. The second is Allegretto but vivace, and is called a Scherzo, so it is quite of replacing the slow movement here. If you want a really "slow" movement, this is no alternative. If you just want a little piece by the way, you should consider it. The "real" slow movement is the Menuetto (Moderato e Grazioso), which would be a good choice if it would not be so boring. But that is just my opinion. The interesting thing about it is the Trio with the Chord jumps. That could be of some benefit.
Op.28 Second movement. Would be my first choice and was my first thought. Although not too difficult technically, it is quite demanding in other respects. It may be difficult for you to play the chords legato in the RH, and the LH staccato, then we have some really nice 32th runs and so on. But the most improtant thing: it is just a beautiful movement! so play it!
Op.27/2 First movement. Well...the classic. Way too easy for you.
Op.27/1 First movement. Interesting, with a Allegro intermezzo. Can become boring after some time, not too hard (although in the Allegro...)
Third movement. Much more beautiful. Has a nasty trill in the right hand while playing some accompainement (but only short). Fairly easy. Very short. The whole sonata is one complex and I speak against playing the movements separately.
Op.26 First movement. A lovely and long Variation set, various technical abilities are needed, very demanding musically as well. Should be in your grasp though (btw, give some examples of your current SOLO repertoire).
Third movement, Marcia funebre sulla morte d'un Eroe. Much easier than it looks, only good if you are generally a bit depressive...
Op.22 Second movement. This sonata was one of Beethoven's favourites, and perhaps THE sonata up to the op.31. However, nowadays it is extreeemmmeelllyyyyyyyy overlooked, unfortunately. The rest is very hard, but the second movement doable, although with some runs and akward passages (holding a not while playing something else in the same hand...). Just play it for the sake of playing this sonata (oh and it is really a nice piece too)
Op. 14/2 The second I think? Don't know really, never played it... sounds funny but looks rather easy for your level(?). Perhaps some chord passages could be of some benefit, so if you like it...
Op.14/1 Second mov. Even easier, quite boring IMO, although it was later on transcribed for string quartet (?). However, don't play it.
Op.13 Second mov. Way overdone, but for a good reason. Extremely easy technically, you could face problems in bringing out the melody, but, so it's up to you, but I think you are beyond that.
So now the first seven sonatas. I have to say, I never played any of them in their entirety, but sight-read to most of them at least one time (That is the reason I started with the late ones. I know them better). Neither did I have extended listenings to them although I try to do that in the near future (kind of discovering the early works again right now). So please take all what I say from now with a grain of salt (and best all what I said UP to now as well
)
Op. 10/3 Second mov. Looks interesting, seems quite demanding (32th! and a rather light touch I suppose), and sounds nice. I have no clue though.
Op.10/2 Second mov. Is an Allegretto, if I recall correctly it is rather easy (that is, too easy for you) but I am not sure...
Op.10/1 Second mov. Most interesting, lyrical on the one hand, but with some awesome runs in between. If you like it, go for it, I say, it is very delicate with many trills and embellishments...
Op.7 The grand sonata, second movement. Personally I like it very much, it is not to long, has a staccato LH like the op.28 and seems not too demanding on the one hand, but not too straight forward on the other...
Op.2/3 2nd mov. Played through it several times, divided in two "sections", one lyrical (easy) melody, the other with the LH containing the melody and the RH playing some tricky (at least that was my impression) 32th accompainment in the treble...
Op.2/2 2nd mov. Largo appassionato (second mov.) Well...again I am not really informed enough to give a valid statement, it seems to have some tricky passages...but then again, it is only a slow movement, so I think you can easily handle any of these. Difficulty should not really be your concern, as long as you take none of the last six. The Largo seems not too easy, though.
Op.2/1 2nd mov. Again many runs and so on. If you want a relaxed piece you shouldn't play this. Nah, well...don't know...
Okay, so, to sum it all up, my Top 5 would be:
Op.28 2nd
(no particular orer)
Op.110 1st
Op.101 1st
Op.57 1st
Op.26 1st
That is just concerning how much I like these pieces, not if they SHOULD played alone (again, I don't like this idea. Play a whole one, probably op.78, op.90, op.26...). These would be demanding but not too difficult for you,
as far as I can see. You should also take into consideration the op.31 package, op.22, op.13, any of the first seven probably, and op.27/1.
And of course before you take one, play through them! Or at least look at the score how difficult they seem and have a listen when looking at the score! (Sight-reading is really useful, you know...) Well however, that was fun. Again, take everything I said with a grain of salt and wait until someone knowledgeable says something.
Topic: beethoven sonata's - slow movements