I would prefer "status quo plus one person is more happy" to "status quo". I would not prefer "orgasmium" to "status quo", because I honestly think orgasmium is nearly as undesirable as paperclips.

If you prefer futures where people are happier as a rule then isn't the best future the one where people are most happy?

Doesn't follow. I generally prefer futures where people are happier; I also generally prefer futures where they have greater autonomy, novel experiences, meaningful challenge... When these trade off, I sometimes choose one, sometimes another. The "best future" I can imagine is probably a balance of all of these.

I don't understand how he could hate being happy. People enjoy being happy by definition.

Present-him presumably is very unhappy at the thought of becoming someone who will be happily a wirehead, just as present-me doesn't want to try heroin though it would undoubtedly make me happy.