in Germany, I'll vote Auerstedt/Iena/ for 1806, Kosen/Freyburg for 1813. Except a few milestone/info panels, the battleground is still "as-was" Iena had a listening post built in the middle during Cold War, now it's stripped to the concrete base and so doesn't marr it
Dennewitz is very good, just a railroad in the middle, but it's completely preserved and prepared for biking it through
Never went to Austria
in general, battlegrounds in ex-East-Germany are intact/or even visitors-prepared (exceptions are Leipzig and Grossbeeren/Blankenfeld which are build over)
Hanau had an american base on it (now turned to civilian housing) and a large cemetary, but the Kinzig is still here (and still unhealthy to wade in late Autumn) and a motorway is at the former wood edge, so it's worth visiting
Bavarian 1805 pre- Ulm battelfields are quite good.
In Spain, Barrossa still has its tower, a compound was built where the British charged, but can be overlooked as it's low lying, Baylen is untouched, Sagunto has a motorway in the middle, but is ok, Arapiles has eaten a little bit of Salamanca but not too much, the Coa bridge is still there and Almeida hasn't grown too much, Vittoria is built over as is san sebastian, never went to Suaroren, but should be good, Maya/Roncesvalles too. I plan to go to San Marcial/Bidassoa/Nivelle some day to check it, but it is heavy forested area, and will need a good map
In Italy, battleground from the beginning of 1797 campaign were signposted for the 200-year celebration, but nothing was made since, so the posts are old a weathered. The landscape didn't change much (1 motorway on overhang…) and looks very much like Switzerland. Did not go further, but cities have grown…