Firstly the last? There are many that are still alive and there will be many more to replace the old and current ones. Each generation has there own frontmen/rock and roll stars that are probably revered and idolised as much as the previous generations have theirs. Although trends do change so there might be fewer these days to choose from when compared with previous generations.
Secondly. Anyone saying Liam Gallagher isn't listening to or seeing anywhere near as much live music as they should. I like Oasis and the Gallagher brothers in general but but fuck me Liams capability is limited to shaking a tambourine in a cagoule. How that makes him, in anyone's eyes the best frontman around is bizarre.
For me the best of my generation (I'm 40) comes from a band that I really like but are by no means my favourite band. The blokes solo stuff has been a bit hit and miss for my taste but when I saw the band live and when I watch this blokes live sets on TV (most recently at Glastonbury) I'm always impressed.
For me you've got to be able to play a few instruments to be considered a good musician/frontman/rock and roll star. Otherwise you're coming across as a bit of a charlatan in my opinion.
So my shout would be Jack White.
From yesteryear I'd say Prince.