I'm always a little bit surprised when Forge gets cited as underpowered for the cost. Back on Iso, I had a gain rate of about .86 with it, wildly higher than most players, and it was one of my highest win-rate cards. (Councilroom won't talk to me at the moment so I can't confirm)
The biggest problem with Forge is that most people aren't exceptionally good at playing with it. You have to focus fairly heavily on obtaining it, and then you have to be very aggressive with the trashing element. Many players are unwilling to do one or both.
Forge offers you very good upgrading (Estate-Silver-Copper into a $5 is typical), good endgame control (via Province -> Province), and some great aggressive plays once you're in full-greening mode (Silver-$5 or Gold-$2 into a Province). Its versatility is great, and it's only really held back by being terminal.
I tend to see Expand as underrated as well (particularly with strong $5s that can be turned into Provinces), but it's undoubtedly overpriced--see Graverobber.