I voted Ralf, but only on the basis that I don't think the title of the thread / poll is actually appropriate - I don't believe many of the drivers in question were underrated - at the time and still now the majority of people knew / know how talented they were. You'll struggle to find anybody that disagrees that drivers such as Alesi, Frenzen or Panis were incredibly talented. A more apropriate title would be something like: 'Drivers of the past that due to circumstances, machinery or other reasons underperformed relative to their known talent'.
Relating to Ralf. He's one whom I believe does fall into the underrated category however. Six wins, nearly thirty podiums for a variety of teams, and in my opinion he seemed to just accept that he was always going to be in the shadow of his brother in his last couple of years hence lost motivation despite publicly sating otherwise. He's not the first driver to have a poor final year or two in the sport (see Damon Hill, Niki Lauda etc. in their last year), and certainly won't be the last, and yet he is often seen as a figure to mock and ridicule by many. If you actualy look at his statistics and race finish positions, under the rules today (when he raced points were only for top 6 and then top

, he would actually have some pretty large points hauls and consisten points finishes that of course were not classed as such at the time - go on, have a look, I guarantee you'll be surprised at the sheer amount of 4th, 5th and 6th placed finishes - I can count over 40 of them alone!! He was never in a position to take a title, and in my opinion had a very respectable F1 career based on all of the above. Others may well disagree and that's their right - but personally I think he's had a massively overly harsh view taken on his career by far too many.
Favourite racing series: F1, Indycar, NASCAR, GP2, F3, Formula E, Trophee Andros, DTM, WTCC, BTCC, World Endurance... etc. etc.