Commission by Fragasnap
Assuming my math is correct, the first Commission only discards one card, which makes Commission just a Lab minus, which makes Commission just a Lab minus in any set that doesn't contain Villages or Villagers. In other sets, however, it's kind of a Fugitive with a bonus, which bumps it up to just strong enough to cost $5. Good job on this one.
Royal Hall by MattLee
Unfortunately for you, it seems like everybody and their grandmother has come up with something almost identical to this.
Royal Heirloom by LibraryAdventurer
This is an interesting take on the "now and next turn" concept. I think it's too weak for $5, though.
Vestibule by Asper
Every other time you play it, it's a better Band of Misfits, but the rest of the time, it's a Throne Room. BoM is one of my favorite cards from Dark Ages, and Throne Room is one of my favorite cards from the base set, so I love this! Fantastic job as usual, Asper.
Collector by King Leon
Holy guacamole, this card is powerful. I think it would be worth $5 even if you limited it to just Action cards; being able to use Night and Treasure cards as well is broken.
Antechamber by Faust
The problem with making a minus version of a card that costs $4 or less is that the gap between $2 to $3, $3 to $4, and even $2 to $4 is actually pretty small power-wise. So as a result the minus card become either too close to the original to bother existing, or it becomes so much weaker that it's brokenly weak. This card is the latter.
Thane by Aquila
This is effectively a Throne Room that can search your discard pile as well as your hand, which is too strong compared to Throne Room to just cost $1 more. You don't even need to have an Action card in your hand to be able to use Thane effectively!
Parade by scott_pilgrim
In most cases, draw up to 6 will be worse than +2 cards, since it's generally easier to have a large handsize than it is to have a small handsize. I don't think players would choose the draw to 6 very often.
Eyre by spiralstaircase
It's a bit of a stretch to call this a Throne Room variant, but I guess it counts. Like others have said, the fairly low chance of high-VP return and the fact that the compensation is just a delayed Necropolis makes this way too weak.
Landgravine by Lurker
While Vestibule does something new and interesting half the time, Landgravine just replaces itself half the time. Not particularly exciting.
Desert Throne by Shard of Honor
See my comment on Antechamber.
Dining Room by hhelibebcnofnena
I feel like this is too much of an auto-buy in Engines that give lots of +$ (enough to buy Colonies/Provinces and still have a little $ left over) and too rare a buy otherwise.
Royal Road by hypercube
I think that this would be useless in sets with no source of +Buys, and too much of an optimal self-combo in sets that do have +Buy. RR-RR is awfully close to KC-Bridge, except you also get to Throne Room another two cards afterwards. (And Throne Rooming two cards will usually be better than the +3 Buys and +$3 you'd get instead if you KC-Bridge.)
Aldermen by ConMan
Renaissance is easily the best expansion we've had for a while, IMO. This card would fit in perfectly with the rest of the Renaissance cards. This is an incredible card.
Appeal by VioletCLM
Appeal feels like it'll be either completely useless or completely busted, with no in between.
Queen's Throne by Chappy7
While it is technically about the same strength as a Throne Room, most of the optimal targets for Throne Rooms cost more than $4, so I feel like you'll be getting the short end of the stick most of the time.
Czar by NoMoreFun
Czar is sort of a self anti-combo, since Czars need Actions to play and Silvers to become stronger, but having more Silver means that you're more likely to draw your Czars dead.
Iron Throne by segura
Selecting one blind card rather than any card from your hand weakens Iron Throne way too much for allowing multiple types to make up for it. I don't even think this is as strong as Vassal.
Sanctuary by 4est
Sanctuary is a pretty sweet self-synergy, since it can clean up your deck to help itself be drawn with other Action cards. It's decent.
Signet Ring by Commodore Chuckles
King's Court is one of my all-time favorite Dominion cards (when I have it, of course
.) I also love Hinterlands, which I think this would strangely fit in with very well because of its on-gain effect. Even though Appeal does it too, an on-gain Throne-Room variant has somehow never even crossed my mind, and this implements it gloriously. Next time I go home for break, I'll be sure to playtest this one with my family.
Royal Library by lompeluiten
I don't feel like this is enough stronger than Throne Room to justify costing just one less than King's Court. Although most cards that even cost the same as KC are not nearly as good.
Solar by Tejayes
I feel like if another player is mirroring you, Solar will be way too weak and whoever has it will just choose the +2 Cards each time, or nobody would even buy it; if nobody is buying the same cards as you, however, Solar just becomes a cheap Throne Room. I agree that it would've been much better with +2 Cards as a consolation prize.
Winner: Signet Ring by Commodore Chuckles
Runners Up: Aldermen by ConMan, Commission by Fragasnap, and Vestibule by Asper.
SUPER, SUPER LATE EDIT: I somehow forgot to add Commission and Vestibule as runners up. Sorry, Fragasnap and Asper.
card. You can't stack it to give out multiple Curses, because of the top-decked Curse. But I think it will rarely give out Curses; unless your opponent already has a strong engine that can plow through Curses, they won't choose that option, and instead the card will mostly read "+
... Transmogrify costs 
and has a similar call effect minus the reaction, but also the call effect is less powerful and less flexible. It might work at 
, though.