Opinions on the best option for withdrawals/opiate addiction

I’m going into a drug clinic to get help today with an opiate addiction, I’ve tried tapering but I just can’t not stick to it, my question is what medication has the least nasty side effects and makes quitting easier? So far over the phone they have mentioned a slow release injection once a month or daily methadone or suboxone. Any advice would be appreciated as I want this to go as smoothly as possible to avoid going back to abusing.