For me -- late-game, mostly Battleships, since they can attain the top speeds (DNs are 1 lower) due to rounding rules, and they've got enough space for stuff like a space yard or lots of cargo (for invasions), and still be able to be survivable with heavy shielding, QR, PDC et al -- I like having large, amorphous fleets that can split temporarily and still be quite viable. Certain speciality ships may be BC (My biowarfare ships don't need the additional space, for instance).
Standardizing on hull also enables refits between ship classes -- I'll often have a DN combat type, because even if I don't build that many of it (compared to BBs) I may CI-capture enemy DNs. So there'll may be missile ships, PPB ships, APB ships, tractor/NSP/repulsor ships, bombers, invaders... a variety pack, but mostly the same size.
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-- The thing that goes bump in the night