Which side goes well with pork belly?

Depends on how you will prepare the pork belly:

You can braise it (for example, Vietnamese thit kho, Chinese red-cooked hong shah rou), serve it with rice, steamed/boiled vegetables, and/or pickles (like leeks, radishes, mustard greens, carrots, etc). Braising liquid is often served as a sauce, which can be drizzled on rice.

Roast it. Slice and serve like Korean ssam (with kimchi) or Vietnamese wraps with lettuce, rice noodles, cucumbers, fresh herbs. Or make vermicelli bowls instead of wraps. Or use it as a filling in banh mi.