I'd say maybe five years? I think with the benefit of experience I'd agree with your father. Their only REAL contenders at the time where The Small Faces (love them too, great band) and The Rolling Stones (which I'm not fussed about.)
The thing that made The Beatles different was John's quirky song writing ability, Paul's song writing ability (and work ethic) George's capability to be an 'all rounder' and Ringo's flawless rhythm (and his smile.) They were the perfect team. I've not seen any band 'hit it' like they did. Live shows were one thing, but in the studio no one can hold a candle to them.
Soloists I'd debate with you, like Cat Stevens, Bob Dylan etc, but bands? Nah, The Beatles are the masters. It's as simple as that.