en çok neyini özledim

 mainly (at most) excactly what i (have) missed.

my try...

Is that a question or a statement?  I wouldve translated that as ´what i´ve missed the most´ or

maybe ´What I missed is the best´  or   ´I missed what the most?"

en çok neyini özledim

´what i´ve missed the most´ = ´en çok özlediðim.....´

´What I missed is the best´  = ´özlediðim en iyisidir´

´I missed what the most?" ‎  = ´en çok neyi özledim?´

neyi already has the suffix (-i) of the accusative with the ´-y´ to smoothen it, so the extra ´-ni´

could relate to something that´s left out at the beginning (Onun en çok neyini özledim),

Obviously this goes with a question mark. 

en çok neyini özledim?

What (of him/it) did i miss most?  My try.

bravo!  nice explanation.  tsk edm!