Tough question. Assuming you meant I had to pick just one... And assuming you mean something that's available right now... I'd have to pick the Pioneer Zephyr I guess.
But the title of the thread seems to conflict with what you asked in the body. The "best" might not be my first pick. I really have no idea of the quality of the Con Cor Pioneer Zephyr so it might not be "best".
I've heard great things about Tower55 diesels.
I have a couple of Broadway Limited diesels that are phenomenal, but they are not as detailed as some would prefer.
By "best" I would choose the newer Atlas 4 axle diesels, based on my personal fleet, and experiance with others.
If I could have any loco on the market, hmmm.... same answer.
I kinda like the Proto 2000 since I have a few of them and they are nice runners and pretty good detail also . If I could afford something better .Probably a Broadway LTD. Or something in Brass!
Hmmmm! probably the one I couldn't afford to buy. A whole lot, I'd say
Seriously I have what I want, I might build some more, but I believe I'd sooner put my $$ into DCC before buying top of the line locos.
Good question though.