Display Enlightment by NCX

eyeing over the Iiyama G-Master GB2560HSU-B1 , haven't seen you mention that monitor anywhere

I'm not aware of any proper reviews for that model, both iiyama and 24.5" TN quality are inconsistent, and all TN panels look awful compared to the U2715H, regardless of how much they cost.  AHVA, IPS and PLS versus TN panel comparisons here.  Save up an extra 50$ for the Asus VG279Q (144hz AHVA panel) or look for the untested, but likely extremely similar Acer Nitro VG271 Pbmiipx which sometimes sells for less.  If these are not an option then consult the Best 24-27" 144hz 1080p Monitors with Free-Sync section of my buying guide which ranks the top tested 144hz 1080p monitors.

Which of the 24" 144hz TN monitors do you think has the best color/contrast in the <250€ price segment?

I don't live in Europe or rank monitors based on pricing (it varies by +/- 50$ too often) unless the company has a history of significantly overcharging for non-professional monitors like Eizo does. 

I was about to buy the LG 24GM79G-B for 212€.

I removed it from the top 10 after seeing these measurements from Belgium Hardware and factoring in the dark scene banding and gamma variance between the tested units.  Belgium Hardware AOC G2590PX measurements.

choose the Viewsonic VX2458-mhd or the XG240R?

Assuming consistency (there's only one review) the VX2458 is significantly better, however, as stated in my buying guide I don't know if it suffers from dark scene banding.  The AOC G2590PX is the safest TN choice since it does not suffer from dark content banding and consistently comes with decent preset gamma. 

Or just get a VA?

If you want a VA panel consult the Best 24-27" 144hz 1080p Monitors with Free-Sync section of my buying guide.  They're fast enough if upgrading from a 60hz monitor and reviewers such as PC Monitors contain detailed analysis of their overdrive versus TN, and reviewers such as Belgium Hardware and PRAD provide pixel response time measurements which can be compared.  The PC Monitors AOC C24G1 review contains comparisons to 144hz TN panels and the Samsung FG series VA monitors.