One of the best. Art is very cohesive and pleasing to look at. Game loop is well thought-out. Upgrades fit the tweaks made to the horde survival genre. Choosing the enemy upgrades is pretty cool.
One thing I don't like is training/kiting gets boring when the zombies only follow you. You only evade new spawns. If some of the zombies straggled/"got bored" that could be one way to interrupt your train circle and make the core loop more interesting for the player as they go in circles for 15 minutes. I do like that you sometimes need to go find syringes and health around the map. Map is just the right size. I just think it needs something to spice up the optimal strategy and make it harder.
Edit: I love the raygun