I don't think any of 'em are flat out better than the other. It's more of a preference sorta thing, since all of 'em are such drastically different take on Modern Sonic. Greeneyes is like, "How many abilities can I give a single character," Modern Sonic is pretty much Boost Formula Sonic plopped into SRB2, and Neo is kinda closer to games like Advance 2 or the earlier Adventure titles, and overall feels more like he was designed as a Modern Sonic meant specifically for SRB2 (In my opinion). I personally prefer Neo because of the fact he feels more specifically geared for SRB2, but that's not to say Modern or Greeneyes are inferior for it. While I don't prefer using him in this game myself, I can't deny just how insanely well Modern was brought into SRB2. I can't believe how spot on he is to how Sonic functions in games like Unleashed, Generations, Colors, and even Rush. Greeneyes is in a similar boat, since it's crazy how just about every possible key was used somehow.