I've refused to drop Secret Chamber. I continue to really like it for the interesting choices it can give, and especially the way it can sometimes leave you better off after an attack. Also, while it's not a strong enough effect to actively play for, it's amusing when it combos nicely with other Reactions.
1) This isn't about effectiveness, so I'll go for Tunnel as most fun. (Edit: unless I can have the Market Square+Donate combo.)
2) Diplomat. Most Dominion cards are straightforward enough that I can remember what they do. Diplomat is an annoying exception.
3) Yeah, Watchtower. Maybe good old Moat comes second, especially on a 5/2 opening?