I did some work on this pack (cleaning up the fighters), so I’m naturally a bit biased, but these are one of the best HD monster packs available if you don’t want to go the MaraToon route; they’re faithful to the originals, but substantially more detailed. Until a skilled artist is willing to make faithful HD sprites, these are probably as good as we’re going to get (or at least tied with treellama’s ML Super Res plugins).

Note: Since this plugin contains hundreds of large images, it may not work on many maps with 32-bit Windows builds of Aleph One, particularly if you also use a lot of other plugins. The crash that enderandrew mentioned occurs because 32-bit Windows apps are limited to 2 GB of RAM – if a level loads too many monster collections, it exceeds that limit. (Most vanilla Infinity levels should load at most four monster collections, and most vanilla M2 levels should load at most three, but a lot of third-party maps use way more.) If these don’t load for you on Windows, get the latest 64-bit Windows app from https://github.com/Aleph-One-Marathon/alephone/releases and run it with that. (This also applies to Eternal 1.2.0 and Juzo-kun’s brilliant MaraToon plugins, among others.)

If you’re one of the <1% of people still running 32-bit Windows: (1) Sorry. (2) Go to Preferences > Graphics > Rendering Options and reduce the Replacement Texture Quality of Sprites until the game loads. (If you also use HD textures and landscapes, you can reduce Walls and Landscapes too, but those probably use far less memory on most vanilla M2/Infinity maps than these sprites do.)