Mr. Foster sure seems to be loved, but my second favourite would be the Pyro itself... if that counts as DLC.
Mr. Foster is just better since he has better features compared to the Pyro itself.
Mainly, Mr. Fosters gas mask is longer, compared to the Pyro's, which is short and makes it seem a bit like a 'gas mask snout'
Mr. Foster's choice of clothing also appears to be more tall and slender, compared to the Pyro who has loose clothing, making it seem shorter, fatter...
There always seems to be a very popular character in every DLC that I see everyone run around as; So, if I had a second vote, mine would go to Paramedic Alfred. As a Medic myself, I find him to be befitting (for obvious reasons), as well as the fact I can't say I've seen anyone use him. Everyone is always so caught up in trying to look so 'awesome, badass and cool' that it's nice to have a different touch in character; A flu mask paramedic which clearly says 'I heal people', a more sincere kinder approach to a character... But, obviously still with some punching power behind it.
(The ones I find commonly used in each DLC pack; DJ Scully, Mike Noble, Reverand Albert)
Heh, just going through each DLC to find those name; I couldn't help but realize that I've only seen one Postmortem character. All the more reason why I enjoy Paramedic Alfred.