Nicotine and sugar! While the latter isn't a drug, it's still worth mentioning due to my severe addiction to it.
Cannabis and Gabaergics should also be mentioned. Cannabis I use everyday in smaller amounts but I do get withdrawal symptoms when not consuming for 24 hours or so, mostly in the form of insomnia, increased sweating during sleep and my already short fuse becomes even shorter.
Gabaergics is only a problem because I have social anxiety, OCD, sleep problems and stuff like that to deal with. It never appealed to me before those problems started to get severe but now I like them a lot. Im mostly good at controlling my use but sometimes end up taking more than what would be considered optimum theraputical use.
EDIT: What is 'hard'?