Wow... N-Card wins all 3 easily really.
* Question 1: What do you think is the best flash cart to buy currently without considering the price?
N-Card because it gets update essentially every other day depending on if there is anything that needs to be fixed. Everyone talks about how good the R4 support team is, but in reality the N-Card support is even better. Not to mention that it is the offical flash cart of the largest Chinese flash cart forum (NDS-BB) so support will be there virtually forever.
* Question 2: What do you think is the best flash cart to buy currently with consideration of the price?
N-Card without a doubt, its true that the R4 is cheap at 30$, however, the N-Card is also 30$ with 1GB of built in memory. Making it more then a bargain. The N-Card is just severely underrated around most places so people don't know this.
* Question 3: Which flash cart do you think changed the scene with its features, which cart was the best of its time?
Again N-Card. First card to have automatic DLDI patching. First slot one to use USB. First slot one to have built in memory. (NDSGBA and NINJAPASS don't count because it just sucks, really really bad) First slot one to have true 100% compatibility. (Downgrade to the first firmware and you will see that all games still boot. The only game that doesn't save is Zelda and a couple others)
Everyone was hating on the N-Card, but in reality it has better game support then R4, better software support then R4, much cheaper then R4.
The R4 is the most popular Slot one therefore its quality is being overrated too kingdom come.