I guess it depends on what amp you're using but with the Mesa Electra Dyne I like the BB Preamp the best. And I use this amp like I would a Marshall, just enough gain from the amp for a crunch sound then the pedal kicks it over the top.
I've owned and tried the Suhr Koko boost, Carl Martin Plexitone, OD820, OCD versions 2,3,4, Suhr Riot, Tim and Timmy, and I like the BB Preamp the best. Level up high and gain down low. The Carl Martin and OD820 are cool though.
Rich, FWIW I liked the Bogner 20th boosted. I thought stock it had decent gain but really not a whole lot, not what I'd call high gain. But the Electra Dyne doesn't have a lot of gain either unless it's cranked and this fucking thing goes from off to LOUD. But the Dyne loves pedals and I liked boosting the 20th as well. Think of it as just more sounds with and without pedals.
The only amps that don't need pedals are the extreme gain boxes like Engl or Mesa Mark amps like the III and IV, then they just turn the amps into Niagra Falls.