Who has the best cap, preferably with cooling.

I like the Max Machine Worx billet top cap. They also have a shower option that attaches to their cap. Merc's XR cap is an improvement, it may also be a decent piece....no experience.

I use Billet Marine's top cap. It has cooling. I think that Thunderusone does too

I also use the 'Billet Marine' top cap as well, has about 8 cooling port outlets around the top and all the water goes to the center first to make sue the bearing carrier stays cool, built in shower input and doesn't interfere with tilt / trim at max up / out.

That and it comes in black so I like it! You'll need longer shaft bolts though.