A normal-to-short-tele prime is usually the best one-lens compromise on Micro Four Thirds. For street and travel, a 17mm or 20mm prime gives a versatile everyday field of view. If portraits are the priority, a 25mm or 45mm prime is often better: 25mm is a flexible all-rounder, while 45mm is stronger for flattering portraits but less useful in tight streets or interiors.

So the best single-lens choice depends on what matters most:

• street/travel first: 17mm–20mm
• balanced all-around use: 25mm
• portraits first: 45mm

Look for a fast prime if possible, since it helps in low light and gives more background blur than the kit lens.