If you want variety, here's the ranking:

• Binance: 350+ coins. Best for altcoins, especially new listings
• OKX: 320+ coins. Good selection, strong for DeFi tokens
• Bybit: 280+ coins. Fewer altcoins but better for futures

Where to find new coins:

• Binance Launchpad — new projects launch there first
• Check "New Listings" section regularly
• Follow exchange Twitter accounts for announcements

If you're into small cap altcoins, Binance is the place. But be careful — new coins can be very volatile.