Tough question, I'm not sure about Minion, but I agree. It's probably as there are many very strong $5 attacks that were released since then.
Clearest choice is Watchtower as already mentioned. Draw up to X got stronger (see Library below) and the trash-on-gain is extremly useful since Dark Ages.
* Scout - it gets worse and worse, but at the same time more popular (this is only half-joking as it's basically an Intrigue-only card and we have Vagrant now)
* Tribute - for basically the same reason
* Moneylender - got worse, it was one of the strongest cards of the Base set is only mediocre now, especially compared to Spice Merchant which is often superior
* Alchemist - not sure on that one, but it shines especially on longer (Colony) boards and Hinterlands made the games faster (Jack,Embassy,FG,...), but even with Dark Ages it's not that often the strongest card on the board anymore.
* Library - also not sure on that one, but the Base Set it was only really shining with Festival or against Militia, but with strong discarding attacks out there and a lot of benefiting cards that discard or strong cards that don't draw like Fishing Village, Warehouse, Oasis, Hamlet, Squire, Necropolis, Plaza to name a few.
* Venture - another one I'm not sure of, but when I started playing Dominion it was much loved and called a very strong card, but it seems much weaker now, probably as the games are more enginey.
As mentioned Dark Ages changed a lot. I like to add Baron (nearly useless with Shelters), Silk Road (less strong with Shelters)