I cannot speak to "best" I suppose, however...
I use a Nikon Rimfire BDC 3-9x40 on my Trainer and have been completely satisfied with it. My rationale is that all of my hunting rifles, including my CZ 550 in 6.5 Swede wear similar 3-9x40 glass. The Nikons offer excellent value in my view.
To help others give you additional input, what kind of shooting are you doing with your Trainer? What are your objectives?
I hope this may be of help.
Cheers