That depends on the number of cylinders, valves per cylinder, and so on. Your parallel twin is probably easier to service (accessability of valve covers, valve mechanism). Some designs require the removal of the gas tank and even camshafts (eg Suzuki V-Strom).

(Old) Ducatis are also a pain with the desmodromic valves (i.e. springless valves). Some of their valves have 0,0 clearance and the mechanic needs to hone down shims to an exact fit. Also the reshimming needs to be redone on older Ducs after only a few thousand kilometers/miles, or annually.

In contrast, BMWs older air/oilcooled boxer engines are a joy to service, easiest accessability (cylinder heads stick out sideways), only 3 tools needed for adjusting valve clearances (two types of Torx/allen keys and a feeler gauge set), 20mins of work or less depending on your routine.