BSK-3 boating Thunderbolt missiles or MRMs are pretty dangerous IMO. I have a friend trying the BSK-1 with TBolt 10s and Silver Bullet Gauss Rifles; I don't know how effective it is in practice yet, but I anticipate the said friend will inform me of their experience when I ask about it next time we play together.
On the Clan side of things, I haven't seen a lot of Makos or BSK-4 running around yet. The IS Bull Sharks seem to be more popular overall. If there is theory to bring a lot more power out of the Clan Bull Sharks, I'm not the person to ask; but if I happen upon some particularly dangerous BSK-M or BSK-4 builds in the wild, I will let you know.
I don't invest in competitive, meta, or otherwise optimized builds outside of comp seasons I happen to be participating in; I personally prefer to play stocks a majority of the time, with BSK-M, BSK-1, and BSK-4 being my favorites by that standard. Also by that standard, I don't particularly love the BSK-2 or BSK-3. The BSK-5 and BSK-6 seem nice in theory, but in stock configuration they have yet to impress me in the way my favorites did.