A benchmark answers the question of whether the result obtained is good. A return of 18% per annum by itself means nothing: if a comparable risk deposit yielded 20%, the result is poor, but if the market as a whole fell, it's excellent.
As a benchmark, choose something comparable in asset class, currency, and term: for a stock portfolio — a stock exchange index, for bonds — the yield of government securities of the corresponding maturity, for conservative savings — the average deposit rate, for any investments in sum — inflation as the minimum threshold for maintaining purchasing power.
On the Uzbek market, benchmark selection is complicated by uneven liquidity: an index calculated from securities with infrequent trades reacts to individual transactions and poorly reflects dynamics. Therefore, for a private investor, it's more practical to compare results against several benchmarks at once — inflation, deposit rate, and government bond yield — rather than rely on a single index.