What are you using as ground cover?

I’m losing the battle against my neighbor’s weed farm (literal weeds like goutweed (Aegopodium podagraria), not even the fun ones) and need some good suggestions for ground cover. I’ve already planted a ton of geraniums and they’re great, but some variation would be nice. I live in Denmark, which means winter temperatures occasionally get down to around -20 oC /-4 oF.