Watford's Most 'Natural' Goalscorer Ever

That volley at Bristol City is absolutely incredible

 

Most natural goalscorer probably MoJo. But that doesn't necessarily make him the best ever. He dissipated his powers by being a bit of a c**t: Ill-disciplined, unreliable and a trouble-maker with questionable loyalty. GT put a brave face on it, but was glad to shift him to Celtic. Several of the players you've all mentioned would be ahead of him; off the top my head I would particularly go for Holton, Blissett and Phillips. Holton scored 48 goals in one season for Watford, (but, of course, that was in the fourth division.) Blissett and Phillips were better all-round players than Johnston, they could be guaranteed to work hard at their jobs and for the team and were both good at goal-scoring.
Vydra scored some great goals in the Championship - really top-drawer finishing- but he seems to be a moody player and coaches don't seem to be keen to use him. Danny Graham was stellar for one season, but wasn't before and hasn't been since: A lower-division Jose Torres. These One-season Wonders are not all that uncommon and there's probably a book-sized list of opinions amongst coaches as to why that happens.
Johnston certainly could finish, though: One that sticks in my memory was at Charlton in the Cup: He was standing in the far corner of the penalty area and a cross was sent over to him - it arrived at about hip height and slightly to one side (and therefore awkward as hell to hit properly). He left-foot volleyed it straight back across into the far top corner of the goal without any apparent difficulty.