There are some indexes
Coinbase has its index.
I don't like coinbase index because it ignores ripple, which had about 5% of market share.
https://am.coinbase.com/Current Composition
Bitcoin 83.00%
Ethereum 10.76%
Bitcoin Cash 2.51%
Litecoin 2.49%
Zcash 0.60%
Ethereum Classic 0.30%
Basic Attention Token 0.16%
Also bitwise investment
https://www.bitwiseinvestments.com/indexes/Bitwise-10They have top10, top20, top70, top100 by maeketcap.
top10 bitwise
Bitcoin 76.43%
Ethereum 8.96%
XRP 5.85%
Litecoin 2.52%
Bitcoin Cash 2.33%
EOS 1.56%
Stellar 0.67%
Monero 0.59%
Cardano 0.56%
Dash 0.51%
All indexes should be compared to Bitcoin performance. Be careful when analysing this past data, because indexes all have a worst performance than bitcoin in the past year . Looking the graphics, 3-5years ago the indexes performance was much better than btc.