Holiday Rentals is one of the best known, so will at least get plenty of hits.
Some sites charge for listing but get very bad google ratings and don't deliver.
Be careful of companies that deal in "ghost" tenants -- i.e. tenants that pay money for your apartment but that you never get to hear about. One UK (Greater Manchester/Cheshire) based company was caught doing this by an investor friend of mine. They had a husband and wife team working in Marbella who were effectively subcontracting from the UK company and who had their fingers in the till. Despite the investor kicking up a row (and getting most of his money), this couple are still operating in the area for this agency.
Even with the good agencies, it is worth doing some auditing. Best audit tools are electricity bills, water bills, and walking in unannounced -- don't get in the habit of letting them know that you are in town until you actually get there.