Brenneke Rottwiels/Blitz are good stuff...so are the el-cheapo Winchester, Federal, and Remington sluggers.
Believe it or not, however...the odds are that your shotgun will like one a little better than the others.
So, I'd recommend buying 5 or 6 different brands/types (I really recommend buying ones that you can easily find, so you'll be able to get them again without a hassle), go to the range, and do a little shooting....then, just lay in a stock of which ones seem to work best in your shotgun.
Now--in my experience, you really don't get much--besides a metric buttload more recoil--for moving up to a 3" over a 2.75" slug; they don't shoot all that much flatter, they don't hit all that much harder (against deer--with proper placement, I've never noticed a 3" knock a deer down faster than a 2.75...), or are more accurate...in fact, I tend to get better accuracy out of 2.75" slugs in my shotguns...about the only way I've found a 3" consistently shoots better is with sabots. YMMV.