Most Successful opponents for Marian Romans?

Welcome to the forum,

Much depends on your playing style, but most here would recommend collecting an army that you would enjoy, even if it loses.
It took a while before my Spanish could beat the Romans.

Something else to keep in mind is the terrain, the lower your aggression factor, better are your chances of defending in home terrain.
Melee in bad going (difficult hills, woods, marshes) will reduce the combat factor of Roman blade and slow movement.

I/7d Early Libyan – Ag1 – Dry
I/47 Illyrian – Ag3 - Hilly
I/48 Thracian – Ag1 – Hilly
II/11 Gallic – Ag3 - Arable
II/14 Kappadokian – Ag0 - Hilly
II/20c,d Ptolemaic – Ag1 - Littoral
II/23a Pre-Islamic Arab – Ag3 - Dry
II/28a,b Armenian – Ag3 or 1 - Hilly
II/30b,c Galatian – Ag4 or 1 - Arable
II/37 Parthian – Ag2 - Arable
II/39a,b,c Ancient Spanish – Ag0 – Arable or Hilly
II/40 Numidian – Ag1 - Hilly
II/45b,c Slave Revolts – Ag0 – Arable or Hilly
II/47a,b,d,g German – Ag4 or 2 - Forest
II/48 Mithridatic – Ag3 - Arable
II/49 Marian Roman – Ag3 - Arable
II/51 Late Judean – Ag1 - Hilly
II/52 Dacian – Ag1 - Hilly
II/53 Ancient British – Ag0 – Arable

Ancient British are by far my fave force, terrible win rate but you feel amazing if you actually manage it.

Thanks for the Replies. I was looking at Tigranes, or Parthia. Here in the states there seems to be a shortage of Cataphracts online; I did find that QRF has them but the picture is not clear enough to see the figures.

Also I wasn't looking to see what armies do well in DBA, although I really appreciate that info, rather which ones hitorically did well against Rome in this period. Rome at this time was a steamroller.