Can anybody suggest me the highest speed DSPs available in the market now. Also, what would be the best for a filer design application a DSP or an FPGA?
This question is too broad and without some more information about the application and the constraints impossible to answer. For actual filter "design" a dedicated DSP is probably not a great choice and you are better off with a high end general purpose processor such as an Intel Xeon or then new-core I7s, Xeons also work well in multiprocessor setups.
Filter implementation really depends on whether you have high-complexity/low-clock or low-complexity/high-clock rate. Best solution for an 100k+ tap room response filter for audio, or a line EQ for a cable modem are totally different. In general for low complexity applications ASICs are faster than FPGAs are faster than general purpose DSP processors.