BO'P is what I'll play if given the option. I like the variety of biomes in it, and there's at least one biome which is unique and ALWAYS massively useful... the Ominous Woods biome where you can hunt Enderpearls at night without ever having to go to the End (very helpful in early-game).
I've tried most of the others and just don't like their variety. I've yet to find one that surpasses what BO'P does. Right now I'm playing in a pack with ExtraBiomesXL and I'm just not excited by the biomes it provides. Some items in the mod are kinda ridic - plant a 2x2 ExtraBiomes fir tree and you'll never need wood again - who needs to spend weeks on Forestry, developing all those advanced species? Just cut down one EBXL fir tree... and I'm also living off the EBXL strawberry plants since they are massively plentiful around my base and since they easily fill me up. So with EBXL you can forget about having to use food mods like Pam's Harvestcraft. To me, these are signs of a "bad" mod. No disrespect to the programmer of it, and don't mean to single it out because all the biomes mods have drawbacks. BO'P's drawback is that it has so many biomes, you will often struggle with finding the vanilla biomes which are still needed sometimes. But I enjoy that aspect and don't view it as a drawback, I view it as a positive - so you have to explore more for a specific biome? Good!