Hey, I am so glad I found this thread. Stages are a big deal for me, and I have a few thoughts. If someone disagrees, that's fine, because it's just my experiences I'm drawing on here.
-I think my favorite stage for Grundy, especially as player 1, is mess hall. That interactable at the beginning is a much bigger risk for them than it is for me, as Grundy's anti-airs and wakeup-though armor options are too good. Better yet, the pig is Grundy's favorite toy, full punishes with WCC and WC MB into big damage. Lack of right-side corner escape plus the mask so close to the corner is pretty good too, if you read them going for an interactable, you either get the interact hit for free if you used df1 to catch them, and if you just hit them into the corner you can enter the vortex. Because of pig's anti-air hitbox and the good ranges on the masks, I like this stage against airdashers, and most people, actually.
-Arkham/Joker cell is fun too, the flower and tvs are so easy for Grundy to use well, but I feel it favors some power characters as much as it does Grundy (maybe even moreso in the cases of Wonder Woman and Superman), not the case for mess hall.
-When the opponent is heavily ground-based, though, I'm starting to enjoy Watchtower. There are always a lot of hurlable robots flying in, good for controlling the ground, and if you can get to the corner Grundy has good restand resets with that left-corner button, which he can get from combos into MB swamphands so easily. That stage kills matchups that Grundy can zone all day (however few those may be), too.
-Rooftop is obvious power v. tool matchup.
Other than that, I don't know too many stages to a serious degree. I have some small impressions, though.
-Wayne Manor is okay when Grundy's on the left side (Car has a huge hitbox, good stage for Grundy vs tool once he uses it up),
-Stryker Yard and Themyscira port are fun on the right corner for that Donkeykong option (pretty good, need to mix with anti-airs to catch air mobility, if I recall there's no corner escape on port, which is awesome),
-Hall of justice, despite some positive things I saw in earlier posts, I don't like at all for power vs tool. Gives the opponent setups with the roomba to break armor when they normally couldn't, and I really dislike that (I hate hall of justice when playing any power vs. tool, but it seems Grundy most of all, bust of heihachi and robawt gives tool chars too many escapes from corner vortex too). Though when they press the roomba sometimes I can keep them in check with MB swamp hands. Power vs. Power, I like it more, though, suddenly the flying roombas keep air guys honest, and they're slow enough that I have a big long time to react with WCC. It's like I'm the only one with interactables, so fun.
I avoid Ferris for the most part. Also avoid Luthor's Lab, and Themyscira regular (those lion heads are doo-doo).
Also, I don't like Stryker's island regular, not one bit. Seriously, Grundy gets no interacts to catch air people (except left corner minigun is really good), there's a tool escape option in the center and right corner, and in the left corner tool gets the minigun, which is really really good against Grundy's armor, and is activated in the air, which means Grundy has fewer options to catch it.
