I would concur wholeheartedly with Matt's comments above.
Given your caveat that you can only attend
one Euro venue, I would suggest that Spa (
Belgian GP) would be the first choice.
Monaco, IMHO, is something of an anomaly. Horridly expensive, congested, terrible spectator sightlines and a usually dreadful race with no passing, even by F1 standards.
Spa Francorchamps ticket prices are quite cheap by Euro GP standards, and its truly the last circuit that hasn't been completely chicane-neutered by Bernie's FIA toadies & minions. A long fast track with great access everywhere, its Europe's Elkhart Lake.
And where else will you match Eau Rouge?
Tim