Lots of this debate can be seen in the Diety and Immortal Civ ranking threads, but I wanted to add one specific point to this thread. Especially, because I think there's a some crossover to the topic that one sees a lot on this board, namely: how do I get better?
As others have already pinpointed (and Jon and Acken are 2 of the people who understand this game the best), if a player is particularly concerned about Classical era rush and also plays on large maps with lots of City States, Greece is a fantastic Civ. It has you covered with UU's, and the CS bonus can stack with the right gameplay. Particularly on Immortal when you can build more cash than the AI (as opposed to Diety AI with piles of cash). And if you build Oracle and Ally early with a Cultural CS, you can go through Tradition and then get to Patronage 3 before the Renaissance, and start building a CS Science engine... which can be a game-long 15-25% science bonus depending on your ability to scout and build allies.
But the reasons why Poland is so dominant a Civ are worth outlining, because they overlap with the issue of what are the Fundamentals to good Civ play.
Earlier in this thread, there's talk about victory times, and as Acken and Jon pointed out, that gets tricky with people declaring things. In general, though, when people raise victory times, we get into the topic of "optimal play." The caveat, as always, is that people can play however they want and "fast" is in the eye each player.
However, in case any newer, less experienced players are reading this thread, too, I want to reiterate what I've written in some of the other threads ("How to get better on Immortal," how to get faster victories, how to get better, et al.):
There are certain benchmarks to achieving the most straight-forward goal of this game, which is A Dominant Winning Position. Completing Tradition as fast as possible, reaching Education, reaching Ideologies, and reaching science levels. You don't have to play this way , but when people start referencing speed of victory and strong Civs qualities, these benchmarks come into play.
With Poland, those extra policies in every era ensure that you'll finish Tradition as soon as possible, allowing for super fast growth in your core 4 cities. You'll therefore hit Civil Service faster (generally), and thus skip into Pikes quickly in case you need defense. You'll also hit Education faster. And of course, with Renaissance comes Rationalism, which you can immediately unlock with Poland and if you time it well, get into Rationalism 2 asap, which is a massive science boast.
If a person is interested in such things, the most reliable way to learn how win Standard speed science victories in sub-T280 is to master these basics. And Poland makes them easier. In general, it doesn't really "matter" what people think is better or underrated civ, but since we have these threads a lot, it's nice to have a good dialogue.
And again, for newer players reading these, you don't have to play BNW with a straight-jacket of one-way play (particularly if you play on Immortal and below, where a good player can meander and do weird things and still achieve sub--300 wins rather easily), but I strongly, strongly recommend mastering the Fundamentals to a strong, Tall empire.
Once you do so, you'll see these debates through a different prism.