Maps online will designate specific planes to be used as Recon. So you'd need early-war and late-war 2 seaters that are commonly assigned as Recon planes to get reasonable coverage.
For Entente both the Sopwith Strutter and RE8 are used but i think the RE8 covers a broader time span and comes up more often for Recon.
Later war is usually the Breguet or Dh4. Yes a couple of maps also use Bristol Fighter (Winter Cappy) but iirc recon on those isn't worth the trouble!
If it were purely for Recon you'd be well served with the RE8, DH4 and Breguet. The latter 2 are also tough, fast bombers and the Breguet in particular has an excellent bomb load.
The Strutter is a great early-war general purpose type. It carries 8 bombs (i think) and can operate as a maneuverable single or 2 seat fighter.
For Central its either the DFW or Halb CL2. I find the DFW far superior for anything that requires climbing or lifting. Its very common in online missions.
Having said all that BSR makes a good point. If you like 2 seater ops, just pick them all up. Likely to be a Xmas sale soon too.