If there were 3 essential electric guitars that a picker should one, it would be as follows;
Gibson ES-335
Fender Stratocaster
Fender Telecaster
Please note that a Gibson Les Paul isn't on the list. LOL
My son owns a Gibson Les Paul that is my favorite one I've ever played but it's still uncomfortable to play sitting down. It just doesn't balance right on your lap.
I agree with the Strat for sure. I've never personally seen the need for a Telecaster myself, as I can get close enough from a Strat for my needs. ES-335 I wouldn't say is "essential", even though I do really want one myself.
All depends on your musical tastes of course. My own "essential" guitar is my Ibanez Axstar headless, which is the #1 guitar I take out to gigs, primarily so I don't smack a bandmate in the face with my headstock (and also to save my failing back).
I've also never found a Les Paul comfortable for me, though it's definitely more balanced than an SG!