EX, FX, FX2, ... refer to different Cloud Scale ASIC types manufactured by Cisco. ASICs in general are specialized hardware that perform packet forwarding functions.
The difference between the various ASICs comes basically to the number of slices, forwarding TCAMs and flex tiles that each ASIC supports.
These are resources that can be combined in what Cisco calls routing modes or routing templates.
And here is the thing: not all combination of forwarding TCAMs and flex tiles in EX ASICs for example are supported with the FX3 ASICS; there is not a better ASIC than others in my opinion, except for the use the case that suits your environment.
Source: Flexible Forwarding Table on Nexus 9000 <br> White Paper - Cisco
I hope this helps.
