Transistor for Broadband Amplifier in X band (11 to 11.5 GHz)

What is the best transistor to use if I want to design an amplifier that should operate on the frequency range 11 to 11.5 GHz?

What is your specification? Different object, different selection.

As Tony said we need more specs from you, specially what sort of power level you require ?

but as a guide for under 10W there are Eudyna devices like the FMM5061VF which will provide an easy 2.5 - 3W out at
9.5 - 13.3 GHz range. The NEZ 1011-3E provides 8W output
For power levels over 10W, you woud generally have to look at TWT's (travelling Wave Tube) amplifiers

I have built/used a number of FMM5061 amplifiers, they are very easy to construct and get working

Dave
VK2TDN