IMC!! Speaking as an IMC/IR and MEP instructor, the IMC rating is by far the most useful first step. A MEP rating can be a good learning experience but it's then tricky to find twins to fly enough to keep reasonable currency. It would also be annoying to be flying an aircraft with significantly more weather ability but without the extra rating to truly operate the aircraft.
The IMC rating on the other hand is a good natural progression from the PPL and allows you to build your flying experience, accurate flying, etc leaving the MEP rating as a later possibility. If you do do a MEP rating post-IMC rating then ensure your test includes the requisite asymmetric simulated/genuine IMC extras to let you use the IMC rating in a twin as well as a single.