Well... let us analyze this for a moment, shall we?
I personally think Sonic and Twilight share a very "SonSal-Type" of relationship. They both share opposite personality traits from one another that make a relationship between the two very believable, fun, and kind of life changing. For example, Sonic is a free-spirit who does what he want when he wants. He always helps others, despite getting no personal gain, and sees action as a reward. Twilight is a smart and down-to-earth kind of girl. Though her sociability is questionable, once you get he to talk she never stops.
If Sonic was Twilight's first friend instead of the main six, Sonic would be able to use his charm and kindness to get her to open up. Sonic would teach Twilight on how to be more relaxed and more free-spirited. But Twilight would teach Sonic to be a little more grounded and to plan ahead more. Because let's face it, Sonic has never been much of a planner.
Which leads me to my next point, If Sonic and Twilight ever fought together nothing would stop them. Sonic's speed combined with Twilight's magical abilities would put every enemy they met to the ground. Eggman, Discord, NMM, Somber, you name it!
So, in conclusion, Sonic and Twilight share an "Opposites Attract" kind of relationship. With Sonic and RD, it's creepy. It's like they're dating a mirror. They have nothing to offer each other and to us besides racing and some snippy jokes. That's it. Nothing more.