As already recommended, your top requirement should be that it is long and open ended. I also suggest that you pick something with a good amount of internal texture and uses extra soft medical grade silicone.

For best results I recommend x-lube. Dunk your masturbator and bottle of lube in hot water for a few minutes before you start.

If money is no option, Bad Dragon sells top quality open ended masturbators with all sorts of different internal textures.