Hmm. That’s a hard question! I have 2 major lenses so far. The 18-200mm and 50 mm f1.8. 
 
Based on depth of field, I’d obviously choose the 50mm lens but there’s no range to it :/
So I’d have to go with a lens like 70-300mm f2.8 but the thing is I can’t really decide on just one particular lens! Every lens is so different :)  
Thank you for asking anon! :D You like photography too, I take it?