What are the best platforms to do project on FPGA and learn RTL design?

 For doing projects on FPGA , first you should decide application of your project like what you want to make using FPGA devices. Based on you application , then you should decide which FPGA you are going to work on as FPGA specific designing tools are available such as :

Xilinx based FPGAs (Virtex series , Zynq , Kintex etc) , tool - Xilinx ISE or Vivado

Microsemi based FPGAs such as proasic , axcelerator etc , tool- Libero Soc.

Altera FPGAs and so on.

To learn RTL design , you can start learning basics from tutorials available on youtube or you can refer VHDL book by Ben cohen for theoritical knowledge. Best way is to practice developing small RTL designs such as counter , clock divider , multiplexer , half/full adder etc.