Yeah - it has to be Biff.
It isn't so much that he 'Is' Saxon, because all the other members have so much quality and input towards the material - it is more the fact that he is everything that Saxon represents: Rock 'n Roll, Uncompromising, No-Nonsence, unique and fun.
Biff is also a rare breed of Metal Front-Man: he is just like us. We can relate to him like he was a bloke that you have a chat and laugh with down the pub. Yet, weirdly, we still perceive him as Rock 'n' Roll Royalty!
Biff is also a very underated singer - his style is far from pretencious - you can tell that he just lets it come out naturally without being too concious of the technicalities, yet he has an excellent technique and range (espicially with recent recordings)!
And, best of all, his voice is unique, yet it hasn't any particular mannerisms that some vocalists require - mannerisms that end up being very annoying because of repetition. Biff's voice is just... Biff! If you know what I mean!
I think Biff would be the obvious choice for most people, so like Mr M Rider I will provide another choice.
Quinny is a vastly underated guitarist, true - and he is a main component in the writing of some of the Saxon classics.
But I will have to go for ex-drummer Nigel Glockler.
The man was perfect for the band. Glockler never over-played - he could hit a solid-groove as well as Phill Rudd, but when the the situation demanded, he could play up to it like Neil Peart. I also reckon that he is the only Saxon drummer that has had a significant influence on the band's sound. Not only has he written quite a few classics, I felt that his style always improved songs for the better!