this is really long, i hope it helps. also this isn't all my opinions (mostly though), i have borrowed some ideas from other people.
Neutral Stages
Yoshi’s Story (YS): Probably the best stage ever for Marth. Its small size combined with Marth’s great range mean that you are almost never out of reach of your opponent. It even has platforms to aid you in avoiding projectiles. The slanted ground at the edges of the stage make Marth’s usually superior edge-guarding (especially the F-smash and D-tilt) even better by allowing you to land attacks below the stage while staying on the ground. The two lower platforms are also at the perfect height for tippering the F-smash, U-tilt, F-tilt, Shield Breaker, and just less than half of Dancing Blade from below. The close walls only hinder Marth’s recovery a little, as they can make Marth’s very vertical up-B nerve racking to sweet spot. But they hinder characters with longer recovery (most notably Fox, Samus (bomb jumping) and Peach) much more than Marth. Overall this is the best in the game for Marth: 10/10.
Fountain of Dreams (FoD): FoD is another relatively small stage and in that respect provides the same zoning advantages as YS. It usually has platforms (always at least one) but some of the platform positions and movements can hinder shffling and provide unforeseen adversity. This stage seems to vary in popularity much more from player to player than any other. Overall FoD is neither a good nor a bad stage: 6/10.
Pokemon Stadium (PS): PS is a larger stage than either YS or FoD. In its basic two-platform set-up Marth can have trouble approaching versus an opponent with projectiles and good spacing. The only advantages on this stage for Marth are defensive ones (the left side of the fire and earth transformations). If you are good at wall teching this is the only neutral stage where you can show off your skill. Luckily for Marth every transformation has platforms, just be careful when the stage is changing, as there are placing that can stop you dead. One disadvantage on this stage is the reduced visibility when characters are below the stage. Marth’s ability to use any aerial after dropping from the edge (without SDing) is made more difficult to use by the inability to see either yourself or your opponent. Overall this stage provides Marth with no blatant advantages or disadvantages: 5/10.
Battle Field (BF): BF and YS are almost identical in on-stage size and are identical in platform arrangement, so Marth has many of the same advantages. The walls at BF are further away. This makes sweet spotting the edge easier for Marth but also means that tippered F-smashes don’t kill quite as quickly and characters with longer recoveries are able to fully use them. But the platform heights provide the same advantages as YS. BF is most notorious for its bizarre edges, which can frustrate any player as any character. Overall an excellent stage for Marth: 8/10.
Final Destination (FD): FD is the only stage in the game that never has platforms or elevation changes. This means that there is nothing to help you avoid projectiles. FD is also a large stage (both on-stage and off-stage), which means getting early kills with the F-smash is nearly impossible and it allows projectile using character (notably Fox and Falco) to find space from which to safely spam. But the flat surface and large size of FD are not all bad. The lack of platforms means no quick escape for your opponent while you are chain throwing. The flat surface also allows Marth to use his good ground speed (both dashing and wavedashing) to the fullest and makes Marth’s shffling much harder to escape. The off-stage visibility means that FD is the exact opposite of PS when you’re off the stage. In addition the way that the stage funnels you towards the edge and the space directly below the edge make this a great stage for recovering with Marth. It provides the best shelter from projectiles (like Falco’s lasers or Sheik’s needles) while allowing you to fully use Marth’s primarily vertical recovery without any risk of ending up trapped below the stage or being killed by some part of the stage. Overall a good stage for Marth against most characters: 7/10.
Kongo Jungle 64 (KJ64): You may not have seen this as a neutral stage before, but according to the latest SBR rule set it is. The most obvious difference between this stage and the other neutral stage is the average height of its platforms making it the most vertical of any of the neutral stages. This particular feature doesn’t help or hinder Marth, the affect the platforms have on Marth’s combos is that late in chain throws on the spacies one of the moving platforms may catch them. The depression in the middle makes avoiding projectiles easier on this stage than on any other. The other aspect of KJ64 that differentiates it from the other neutral stage is the ability to recover through the stage. This doesn’t help Marth and the resulting edges are actually more difficult to sweet spot than most. What it does mean is that (when combined with the moving barrel cannon) you should not assume an opponent is dead until they die. In terms of size KJ64 falls between PS and FD. Overall a decent stage for Marth: 5/10.
Dream Land (DL): The last neutral stage is by far the largest. With good DI and a little luck many characters (yes that includes Marth) can survive to upwards of 200% on DL. But this may be Marth’s only advantage on this stage. Peach and Samus will benefit the most with respect to their recovery and this stage should not be counter-picked against either of them because of that. The wind can also be a detriment for Marth because it can mess up spacing (especially while edge guarding) though it is not usually a factor. As was the case with FD, you will rarely (if ever) get a low percent kill with the F-smash. Opponents who want to spam projectiles will rarely be at a loss for space in which to do so. And although DL has the same platform configuration as BF and YS, don’t be fooled, you should not use these platforms to tipper. While you can tipper the U-tilt, F-smash, two parts of Dancing Blade, and the U-smash from below, all but the U-tilt (which requires merely good spacing) require precise spacing that make it impractical. Overall this stage is on the low side of neutral for Marth: 4/10.
i haven't really looked at the counter pick stages, but the addition of princess peach's castle to list of possible counter picks is something that every marth player should take notice of. it was originally banned partially because of marth.
. There isn't too much that can be said except that at the ends it has close horizontal kills which are nice but it's hard enough to land a solid F Smash (or any other forward attack...). I'd say compared to the rest of his stages this one is on the low end 2-3.