Which german dialect is easier to know? "Übringens spreche Ich hochdeutsch"
Hochdeutsch is what is spoken on TV (except for "Mundart shows", that one character whose region is important or maybe reality show contestants). So yeah, that'd be what you'd learn. We learn it in schools, we all understand it, and most can speak it, especially in cities. That doesn't mean we always do, unless we're from Hannover, but if we know it's someone's second language we won't speak too weirdly. ;)
I speak heavy Swabian around my family, sometimes it lasts a bit after I've seen them, but I can switch to Hochdeutsch and speak it with my friends. Even foreign exchange students had no problems understanding me. Like everywhere dialect are a bit heavier in smaller towns and those living their whole lives there and/or not planning on leaving. Though a lot of major cities like Munich, Stuttgart, Köln, Leipzig or Berlin also use dialects, most can speak Hochdeutsch.
And which one is the easiest to know. Except Hochdeutsch. No idea. I think they'd all be pretty difficult to learn, since it changes from town to town and despite there being some grammatical differences and difference in the vocabulary they are not a different language - the rules are not really written down and there's no fixed spelling. Plus the intonation is a bit different at least that's my theory on why it carries over after I visited my family, I still have the Swabian intonation and use it on Hochdeutsch and sound Swabian just by doing that.
No, in Freiburg it's Allemanian/Badisch. But there are some similarities in grammar and words (although no one would admit it since there's some well-meant bickering between the two parts of Baden-Württemberg - the Bundesland it's in. Swabian is spoken in Württemberg).
But only people who moved to Freiburg from somewhere else will look at me funny, if I offer a Teppich (literally carpet) instead of a Decke (blanket) to someone who's freezing.
That's a common misconception. Bokmål and Nynorsk are written variants, not spoken ones. The Oslo dialects in the east map onto Bokmål well, but there's lots of variation in Bokmål, and even places that use written Bokmål speak quite differently than how they write (ex. Trøndelag). Nynorsk was compiled from a hodgepodge of various western dialects, and it doesn't really map onto any one in particular.