[BillCar]

I really like Hitman's Optics mod. OLC 1.2.7 is excellent as well.

Hitman's mod has very realistic periscope views. I don't even bother looking at the ship recognition manual anymore (except for when I need to use the fixed wire method to get a precise speed of a ship – always handy to have some paper around for that!), I've gotten so good with just rangefinding using the attack scope hash marks (it's even easier with the observation scope marks). There is no stadimeter in his mod.

I also use:
A ton of the GWX package mods
Torpedo damage Final
Lifeboats and debris
Real depth charge
Realistic torpedo
WB Fuel efficiency / decks awash
Das Boot clothes
No medals / rank on officers
Pisces' mod for having protractors available on all navigation tools
and several more, but those are some of my favourites.