To add on to Patty's answer, I've found that Creeper is a great easy map to grind and that 3Rainbows is a good medium map.
Droppers are also criminally underrated, Pickles can be completed in under 10 seconds per try and if you're very efficient, copy/pasting commands and perfectly setting checkpoints, you can get 50+ mana per minute (if I remember the mana payout correctly).
Lastly, the champion maps are likely the best way to gain mana. If you're at least Skilled rank, you can get 1,000-3,000 mana every 2 days by completing the champion maps Winter Wonderland, Haunted Manor, and Aquatic Adventure.
Aquatic Adventure is very challenging and unforgiving as checkpoints are sparse and the stretches of parkour are long; I haven't been able to complete this map thus far.
Haunted Manor is quite good, but relatively confusing; there are several Youtube tutorials that can help you navigate the map, which takes roughly half an hour to complete (for me, anyways).
Winter Wonderland, though, is by far my favorite out of all of these. I'm partial to the winter-themed build, but the parkour is relaxing and simple for the most part, and you can complete the map in just 10-20 minutes with enough precision and skill. Good luck with grinding!
My personal favorite grinding map is MandMs on hard. It's doable in 30-40 seconds and gives 60 mana per attempt! That's about 7000 mana per hour. Another great grinding map is Luigi, an insane map. Though it can be quite difficult when getting sued to the map, once you get a feel for the jumps, it is consistently doable in 2-3 minutes.
The only issue for droppers is that yes, they give lots of mana when you're starting out, but no score. Once you get to Ninja, you'll be stuck. Of course, champ maps are the best way to grind score quickly, but it can be annoying.
If PK ever resets, once I'm skilled I'd be living on Haunted Manor.
Adding on to my earlier point, Pizza and Creeper are both good easy maps and Pikachu is a good medium map for grinding. Not sure about hard maps, but Random is definitely the best Expert map to grind. As Katrina said though, champions are best.