A few pointers:
1. Synthesizer choice: Generally free VST pianos sound awful unless you know how to mask it. For example, this piano melody will sound better on high octaves. And if you were doing harmonies with the piano, they would sound better stretched out across the low end. Your melody is in that awkward middle area. You can either find a different synth or push it up an octave or two.
2. Tip on writing melodies for hiphop: You need to be able to bob your head too it. So when writing melodies, maybe hum or whistle something random that makes you feel the way you want the song to feel and make sure it's got a good flow (rhythm).
3. Personal preference: That second synth that's accomodating the melody, it seems a little out of place. Maybe again, change the synth, or EQ out the highs so it sounds smoother.