I want to design FIR and IIR Filters...

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which FPGA is the best?

Cyclone

Arria

or

Stratix

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It depends on what 'best' means to you?

They can all implement FIR and IIR filters.

What kind of implementation were you planning on? Are the coefficients optimized to be simple right-shifts, or are full multipliers required?

If you are using multipliers, how many bits do your multipliers require?

What is the sample rate of your filter? How many coefficients do the filters have?

If you can answer these questions for yourself, then you can look at the data sheets and see that the higher-end devices have better multipliers (more bits) and what density you need to meet your bandwidth and multiplication rate requirements.

Cheers,

Dave