The infinite is an afterthought, this is arguably ROB's toughest match-up regardless, easily tougher than G&W (who just seems to get easier as time goes by...).
To quote a certain someone else, ROB's blind spot underneath is "the size of the moon" and ZSS is simply the best character in the game at taking advantage of it. Once ROB is put into the air, she's quick enough to stay underneath ROB as he jukes around and her vertical range is god-awful ridiculous. The horizontal range on her USmash and UAir are deceptively large and are both difficult to air dodge through due to their length and speed, respectively. Going to the ledge doesn't do a whole lot of good either, ZSS's ledgeguarding from the stage is every bit as bad as Marth. Hell, at high percents, if she chases you in the air with Up-B... just let her have it and guarantee you beat her back to the ground. :/ (And tech if you can!)
Playing a ground game is the only option ROB has in this match-up. Unfortunately, ZSS can pressure ROB with her spacing at the mid-range level ROB would start using projectiles against most other characters. Her speed beats ROB in close (1-Frame Jab, ******** priority UTilt, and whatnot) as well, giving ROB a very narrow range which works to his advantage.
Anyway, to address some other responses here... About recovery:
ROB sure isn't worried about ZSS's Side-B hitting during his Up-B. No, ROB can easily recover much higher or lower than ZSS can place it. In fact, ROB should have little trouble recovering low to reach the ledge, as his UAir will eat through ZSS's aerials if she comes off-stage and he can simply sweetspot the ledge if she tries to space something from the ground (DSmash the most likely candidate). However, once he's there, he's **** well stuck there. Spacing the Side-B or DSmash against ROB's ledgehop game is a joke. Ledgejumping is also risky business since it's going to put ROB directly above ZSS, exactly where he does NOT want to be, and she's too fast to just recover over the top and land behind her. Getting to the ledge is easy, it's getting on-stage that's a problem.
Calling ZSS's recovery "very solid" is an extreme overstatement. It might be "better than most people think" but it's hardly impressive and ROB should expect, if not NEED, gimp kills on a regular basis (at least once per match). A Z-dropped or down-thrown Gyro will flat-out knock ZSS right out of her tether, setting her ripe for the gimping as she pops straight up into a weak FAir and shoved back to her death. Her Down-B is easy to read-and-react against, as ROB can hold position with his Up-B as she nears and blast her out of the apex with a BAir, outside of the range she can successfully flipstool or kick out of it. (In fact, BAir will pretty much beat everything else too.) Simply hugging and waiting for her to approach the ledge will absolutely
not work but the ball is pretty much in ROB's court here.
Remember that even dsmash outranges ftilt (at least according to Snakeee). ROB's only recourse in this match is to run away, but ZSS's speed and aerial mobility make this tremendously difficult for ROB to do for long, and ROB's zoning options range from neutral to terrible, with really no advantageous spots.
ZSS also possesses two stellar CPs against ROB in Norfair and Brinstar, so if you ban one, she'll just take you to the other.
Just based off memory, I'm pretty sure ZSS's DSmash does not outrange ROB's FTilt. Regardless of whether it does or not, the important thing is that ZSS has extreme disjointedness on her side. Where ROB can throw out "near-miss" FTilts to zone some other characters, doing that against a ZSS is going to end with him punching a DSmash or Side-B hitbox and getting punished. At least ZSS won't start the infinite from a DSmash that hits at ROB-extended-his-hurtbox-with-FTilt-and-got-hit-from-halfway-across-the-stage length, but still. -_-
Norfair, if it's allowed, should pretty much be auto-ban for ROB. Take me to Brinstar all day and I'll be thanking you for not picking Battlefield or even Yoshi's Island; the fly-through floor, if not a route of viable approach, is a welcome escape route from ZSS's otherwise domineering ledgeguarding. The floor can also be broken up to give ROB a potent camping position on one side of the stage. On the flip side, ZSS is extremely exploitable on certain stages herself, such as Japes.
Anyway... anyone want to recommend counterpicks (stages AND characters) and bans?
Ban Norfair if it's allowed, otherwise ban Battlefield or wherever you feel least comofrtable, I suppose. Japes is an LOL CP against her because you can camp the side platforms all day as the ledge there completely destroys her ability to Side-B pressure you off it. Else, I'd say maybe...
Orpheon -- The first transformation has the ledgeless right side for easy gimps. Just camp the side platforms on the second half all day and she has no way to get under you.
Rainbow Cruise -- Camp the bottom/back end of the screen during the scrolling sections to protect your underside and make her risk a gimp or walk-off kill by approaching.
Lylat Cruise -- The uneven ground and tilting of the stage can screw with her Side-B usage but it doesn't help out much otherwise.
Characters, no idea. I hear Snake and Falco are supposed to be her toughest match-ups but I'm not sure if the new DSmash tricks have changed ZSS players' opinions about that.