The smoothest and blackest writing hybrid ink refill is the Schmidt Easyflow 9000 and it fits into all Parker compatible pens and there are a lot out there. If you want a gel then I suggest Pentel Energel. If you want disposable pens you can get the Pentel Energels as cheap as $1.33 each. TheMonami FX 153 are $2 each and accept the Schmidt EasyFlow 9000.

Want a better pen for Parker refills then get a Rotring 600 (prices change everyday/hour and you should be able to get one for ~$25) - too much then here is a $7 alternative and for a Pentel you can get the business version (metal) for $9