Just my opinions I wanted to share about each stage. For the sake of reference I'm just gonna use the default starter/counterpick designation. Pardon me if it's large.
Legal (17 total)
Starter (8)
• Battlefield
• Big Battlefield - At this point, it can be legal, but if it's too big for some reason, it can be legal in 2v2s and above. From what I've seen, the bottom part of the stage is lower and the camera is zoomed in more than the regular Battlefield to compensate for the amount of players.
• Final Destination
• Congo Jungle N64 - This was mainly banned in 64/Melee for being too large in singles, but afaik it was legal in Melee for doubles but rarely used if anything. Character models are bigger in Brawl/Smash 4, this stage has good platform locations. Possible starter, good counterpick.
• Skyloft - Basically a better Delfino Plaza. Larger main platform and no water.
• Lylat Cruise
• Town and City - Slightly larger variation of Smashville with differing platforms throughout.
• Smashville
Counterpick (9)
• Mushroom Kingdom U - If it's still the same from E3, it can be a good counterpick; possbily a starter on liberal stagelists.
• Delfino Plaza - It was mainly banned in Brawl for exploitable techniques from characters, like MK with sharking and D3 with wall infinites. Neither are problems now; it can be a legal counterpick again.
• Mario Circuit 8 - We've seen this track enough to designate it as legal. Good main platform, unique dynanism, less interfering Shy Guys than other stages, few walkoffs.
• Norfair - The main problems for this stage in Brawl were the ledge invincibility and gliding characters. Both are gone, and the hazards are easily detectable before they appear. Counterpick possibility.
• Halberd - A good all-around counterpick with its low ceiling and generally small estate, and good drop-off platform with detectable hazards.
• Orbital Gate - I personally see no reason why this stage should be banned. We've seen enough footage of it to be a good counterpick. The hazards do small knockback and easily noticed before it happens, and it's easy to understand where the platforms will appear without knowing about the stage too much. It keeps the fight going. Counterpick, honestly.
• Castle Siege
• Pilotwings - Good counterpick with varying stage angles, rare stage hazard.
• Wuhu Island - Basically another Delfino Plaza with few walkoffs and swimming.
Questionable (12)
• Mario Galaxy* - Walkoff. Contrary to what most people think, the gravity gimmick here scales accordingly to the plane, so you can think of this stage as if it was still completely flat. It still affects hitboxes slightly, though.
• Luigi's Mansion - Awkward platform placement and cave of life, but this is a different game, it has a chance still.
• Port Town Aero Dive - The main reason this was banned in Brawl was because of the ungrabbable ledges on the main platform; it heavily favored characters with good aerial mobility. If the main platform has ledges, it can be an effective counterpick, but the cars might still be sketchy to some areas unless they were nerfed a la Onett's cars.
• Woolly World - I personally think it can be a counterpick. It's a stage with the most unique platform placement and a somewhat long walkoff transformation; it can help promote variety. The platforms might favor characters with aerial mobility (Peach/Jigglypuff) but again, it's only a transformation.
• Kalos Pokemon League - I also think this can be a counterpick. Rayquaza is not shown to interfere in the Dragon Chamber, and it has a 17% predictable hazard. Fire Chamber has the fire pillars that do minor dmg/knockback. Water Chamber has a short stream of water for 2.5 seconds and a walkoff. Steel Chamber has swords like fire pillars that do small dmg/knockback. Depends on the TO, honestly.
• Coliseum* - A good stage, but has a walkoff.
• Gamer - Mom is a fairly powerful hazard albeit incredibly predictable and you have ample time to adjust yourself, and likewise, your opponent can effectively hit you into 5-Volt's ray of sight. If 5-Volt was on a timer, it could be legal, but she seems random. The platforms also change every time the stage is picked. However, 5-Volt only kills about >90%. This stage can be a counterpick in the most liberal stage lists.
• Garden of Hope - The crab is very powerful and somewhat large and moves fast across the stage. Needs research first. It'll either be a counterpick or banned, unfortunately.
• Wii Fit Studio* - See Coliseum.
• Onett* - Needs experimentation.
• Wrecking Crew - Interesting dynamic like Gamer but not exactly a hazard like 5-Volt. It's similar to Tomodachi Life, if anything. Needs testing.
• Windy Hill Zone - It's hard to judge this stage based on footage. It seems large. Singles is iffy, but doubles and above is fine.
* = Walkoffs can be either-or at this point because of Smash 4's modified engine.
Banned (14)
• Mario Circuit (Brawl) - Walkoff with awkward platform placement, the Shy Guys are more intrusive than Rainbow Road and Mario Circuit 8.
• Jungle Hijinx - There's a bit too much going on in the front plane. I don't think the two-plane fighting is good competitively. At least Sakurai scaled the knockback to account for the background plane's largeness.
• Bridge of Eldin - Too large. Walkoff. Hazard is decent but overcentralizes one part of the stage, promotes camping, and encourages faster characters.
• Temple - Probable in quartets but otherwise still banned for problems in the past. Cave of life, circle camping (Peach and Jigglypuff), etc. Still the funnest stage if anything.
• Pyrosphere - Ridley.
• Yoshi's Island Melee - A bit small, awkward platforms, right-side walkoff.
• Great Cave Offensive - No explanation needed even if it's fun.
• Palutena's Temple - See above.
• Skyworld - Breakable platforms and favors certain characters with better recoveries because of the ledges.
• Boxing Ring - Walkoff, camping, large, overcentralizing lamps and main platform.
• Gaur Plain - Metal Face.
• 75 m
• Wily Castle - Yellow Devil.
• PAC-LAND - Sidescroller and instrusive layout. Above all, it's ugly.