there is no top online wallets because everyone prefer a different wallet , and about me and the most of people and who use bitcoin prefer blockchain or xapo for my experience because of the security , low or no fees , fast transaction , available in android or ios like an application easy to use in the phone
but maybe there are some people who prefer others wallet because of market exchange how is not available in blockchain and xapo
Most important aspect of web wallets is that they should supply you with the private key belonging to your wallet address. Other features come after that.
In case the wallet service in question experiences downtime or other problems, you'll still be able to gain access to your coins where you can quickly move them to your own wallet for example.
If a wallet service doesn't give you access to the private keys, then it should be a straight no go. There are enough alternatives.