I would like to tryout some long term strategies on SPY. What broker would be the best bet for a paper account?
It's paper trading. In terms of results, it won't matter much which broker you use. A better approach would be to determine which broker best suits your needs (commissions, trading tools, margin, etc.) and then use their simulator. That will enable you to get comfortable with it and you'll be ready to go when you hit real time.
Try tos (thinkorswim.com) and see if it suits you; their backtesting interface is pretty good FWIW. -gariki