The European session in forex runs from 7:30 am to 3:30 pm GMT, while the City of London is within business hours. This is the session that experiences the most volatility but is most active when it overlaps with the other major session hours.
The best pairs to trade in the European session will depend on your strategy. Ultimately, the sheer liquidity of forex trading that takes place means that almost any pair can be traded. Typically, you find the most movement among the major European currency-based pairs, which include EUR/USD, GBP/USD, USD/JPY, and USD/CHF.