Ohh. That's tough. I mean twenty years is a long time. If you're 18 at the time then it'd be more than double your lifetime later, so it's really hard to tell. I'd probably at the least hug, and suggest catching up.
Depends where I'm at and who I'm with. I wouldn't do any of the listed suggestions. I'd probably say hi and strike up conversation. See where we're both at. See if the spark is till there.
I'd hug and go for a drink with him, see how he is etc. If I were single, and he was single, yeah sure I'd probably see if there's a possibility of getting back together. But if I were married and happy, I'd hug and go for that drink and invite my husband along. I do have one ex with whom I'd always wondered what would've happened, and I find every time I talk to him, I still feel like I could fancy him again. I haven't a clue what draws me to him, because I know we're not good for each other. I'm happily married. So, I'd personally be quite careful not to keep in touch too much. But I'm the sentimental sort - he was my ex and therefore once upon a time he was very important to me, and that doesn't change for me, to some extent. On his part, I know he's moved on well and truly, which is exactly as it should be We would be pleased to see each other again I think, but because he's my ex, and because I know there's a chance I could get emotionally attached again, I would never make the effort to visit him. Interestingly enough, my husband's actually more affected by an entirely different ex. But I know which ex I need to be more cautious of lol, and I wouldn't give my husband up for the world.