FarmGrid ist a must-have for me, because I get frustrated with the vanilla planting system very quick.
Other than that: Some HD-textures ( I like willybachs most) plus other mods that enhance graphics or performance, like render limits and custom graphics.

Performance of this game can be somewhat bad, because by default it only uses 512MB vram max(and probably because its not optimized yet), you might want to increase that....that is what the custom graphics mod is for.