I've enjoyed all 17 of them to be honest. Favourite grounds have probably been Koln, Monchengladbach, Frankfurt and Schalke. All brilliant atmospheres.
Was at Hertha last year when they beat Bayern which was also pretty special.
Bizzarly, I found Dortmund a bit of a let down to be honest.
Other grounds I've been to are Hamburg, Mainz, Leipzig, Wolfsburg, Leverkusen, Kaiserslautern, Stuttgart, Hoffenheim, Union Berlin, RW Essen and Fortuna Dusseldorf.
Next week my mate has decided to hire a car (first time doing this, normally just go by train so travelling a bit more than usual / have more flexibility).
Thursday Gent v Saint-Etienne
Friday Schalke v Mainz
Sat I have tickets for Bremen v Leipzig but keeping an eye on Bayern ticket portal for Bayern v Koln
Sunday Frankfurt v Dortmund.
So returning to two grounds but they are two of the best in my opinion.