The 50f1.8 was made in several variations over the years. The last two (marked "Made in Japan" and "Japan") are considered the best. I have tested my MIJ against an earlier model 50f1.8 and it is definately sharper. If you want small, the 40f2 is likely the smallest fullframe 35mm SLR lens ever made. Some folks have been disappointed with the image quality, but mine is very sharp with great color and contrast. All three 28's are well respected, and they give you a free puppy when you buy the f3.5. If you like zooms, the 35-70f3.6 is the best in the range, but large for a Zuiko (I'm being practical. The 35-80f2 is widely considered the best zoom ever made by any company for any camera, but you'll never find one.) The 35-70f3.5-4.5 is well respected, and hardly bigger than a 50mm prime. The 90mmf2 Macro is one of the best lenses you'll ever find.
The list could go on and on, but it is rather pointless without knowing what kind of shooting you do, whether you prefer primes or zooms, etc. Get a lens that suits your shooting style. They are all Zuiko's, they all produce "Heavenly Light".