ISAB released a video recently rating maps on how interesting they are: https://www.youtube.com/watch?v=bXUfcueR4cA
I wanted to make a ranking of maps by difficulty. That's different for everyone, based on your strategy. My main strategies are Obyn/Druids, Striker Jones/Biggest One/MOAB Eliminator, and Sub Commander/Prince of Darkness, so that affects my rankings. The original game rating of the map is indicated in parentheses.
I'd love to see someone else's rankings. These are mostly based on CHIMPS mode.
Easiest to Hardest:
Logs (B)
-By far the longest map
Frozen Over (B)
-Great water placement, long map, tons of self-intersections and straight lines.
Cubism/Carved (B)
-Same as above with slightly weaker water placements.
Four Circles/Park Path (B)
-Very long maps, but without straight lines. Park Path is perfect for 0-3-2 Monkey Aces to drop bombs on.
Monkey Meadow /Winter Park/Town Center/In the Loop/Tree Stump (B)
-Bad (or nonexistent) water placements and poorer loops.
Alpine Run (B)
-Harder than it looks. One of the first maps to pose more of a challenge.
Streambed (I)
-The only reason this is Intermediate is because of the removeable object. The first loop is great for sun avatars or druids.
Karts and Darts (I)
-Another map that's only Intermediate because of the removable obstacles.End of the Road (B)
Cracked (I)
-Would be really easy but a lot of the corners are too small to fit much in.
End of the Road (B)
-The shortest Beginning map. Requires a bit of a different strategy.
Adora's Temple (I)
-The height placement is a bit tricky, but everything loops around the entire map once before going straight. The straight edges and tight corners are great for powerful towers, and The Biggest One is amazing.
Hedges (B)
-So obnoxious! Line of sight is blocked everywhere, and the weird corners mean your towers are missing half the time. Airplanes and helis should make it easy but somehow it's not? Took me many many more attempts than any other Beginning map.
Moon Landing (I)
-Perfect for mortars and helis. Rough for most other things.
Downstream (I)
-The river really throws off the best placements, and the two loops are too far apart for most towers to reach both.
Firing Range/Spice Islands/Spring Spring (I)
-Each of these introduces placement issues: Firing Range requires long-rage towers, Spice Islands limits land towers, and Spring Spring splits most the track in half.
Cornfield/Spillway (A)
-These are advanced maps, but pretty easy. Spillway is perfect for The Biggest One, and Cornfield is a long map with tons of corners.
Haunted (I)
-Like ISAB says in the video, having entrances and exits in the same spot makes it hard to see if anything is sneaking by.
Off the Coast (A)
-Not that bad with any viable 'standard strategy'. Standard strategies start becoming a lot more important in the advanced maps.
Dark Castle (E)
-Maps with one exit are a lot easier than other maps.
Chutes/Rake (I)
-Both of these have extremely difficult early games. And these are the first two maps in this list with two tracks that are mostly distinct.
Another Brick/High Finance/Underground (A)
-Not a lot of room, and lots of lines of sight blocked in the first two. Short map in the second.
Cargo (A)
-Short, with different tracks for blimps and bloons.
Pat's Pond/Peninsula (A)
-These were so hard. Both seem designed for water towers, but Pat's Pond blocks all good water spots early on, in addition to having the exit and the entrance being the same place. Peninsula is very difficult to place heroes on, and using an ice tower to make more land takes up a ton of money.
I haven't really been successful on the other Expert maps, but I can say Bloody Puddles is harder than Muddy Puddles! I'd probably rank them as:
Muddy Puddles/Workshop (E)
-Both of these are one exit at a time.
Ouch/Quad (E)
-As ISAB points out, Quad is #Ouch rotated 45 degrees.
Bloody Puddles (E)
-Only recently possible.