I sounds to me like what's being asked here is not what "the greatest card" is but what "the most powerful effect or combination of effects" is that is printed on a single card-shaped object.
The list obviously ignores cost. If we are taking coin/debt/opportunity cost into account, then the greatest card in Dominion is Chapel, debate over.
When listing Champion, Teacher and Prince, my assumption is that the card is already in play (aside or tavern mat) ready to do its thing. When listing Donate, my assumption is that you are imagining a scenario in which you could just Donate whenever you wanted without regard to cost, because cost is not being taken into account in this rating.
So, starting with those assumptions, I have two comments.
First, I want to give at least some props to Teacher. Champion's immunity to attacks probably puts it over the top, but my thought is that often, you only need to +action a single key terminal action card in order to make an engine hum, not ALL of the terminals. The ability to also add +card and +coin is extremely powerful. There are very likely many kingdoms where free Teacher would beat free Champion. But attack immunity is a deal-breaker. Advantage Champion. That said, I'm not sure the top effect(s) is one of those two guys(gals) anyway.
Including Dominate raises an interesting issue: VP effects are extraordinarily hard to rank outside of their opportunity cost. If Province cost $2 it would be far more powerful than Dominate, because the pile would empty long before anyone could Dominate. If Dominate was free...let's see...the most powerful card in the game would be, what, Squire? Ranking effects ignoring cost is starting to seem silly.
And including VP effects raises the same sort of issue as ranking Chess pieces: Is the the Queen or King "greater?" The Queen is a more powerful attacking piece, but the King is the actual victory condition. If the direct victory condition can be included in the ranking, the King is infinitely more important. Therefore, Dominate, if included, is the single most powerful card-shaped object in the game, because who ever has the most of them wins, period, regardless of everything else. For that reason I would suggest omitting anything VP-related from consideration (and if that's not a slippery slope...)
Secondly, I want to give extra props to DX for outstanding overall game design. Look at what just happened. In a discussion about the most powerful effects in the game, the majority of the cards don't actually do anything! They're all cards that make other cards better directly (Champion/Teacher/Prince/King's Court/Fortune) or get rid of junk so that other cards have more of a chance to shine (Chapel/Donate.) It says a lot about good game design when synergy is this fundamental and no single card wins games all by itself (even Goons!)