I want to heap praise on a person. He is a very hard-worker and works most of the time of a day. He takes a great workload and is always eager to take further workload. His tasks are quantitatively and qualitatively appreciable. He never complains and keeps on discharging the jobs with a smile. I want to devise a catchline, much like what most of the advertisements today use, for him. The catchline I devised is "... a tireless workaholic". (I know that "workaholic" has a negative shade and is used mainly for those who keep so engrossed in work that they have no time for the other aspects of life. But, I find no better word either. So, I have settled for "workaholic". Now, I want to change the shade of the word by qualifying the word with a positive adjective. I find no better adjective than "tireless".) First, I do not know if the catchline is grammatically correct. If correct, I do not know if, for "tireless", a better adjective could be substituted. Please help me.
"Workaholic" is indeed not the best word because of it's negative connotations of compulsiveness and lack of time to do anything besides work.
I would use words like "dedicated", "committed", or "hard worker" to describe that person:
committed: willing to work very hard at something SYN dedicated
He is extremely committed.
You could consider using "respectable" before "workaholic", which means:
Regarded by society to be good, proper, or correct
[Oxford Online Dictionary]
(1) Worthy of respect: estimable, (2) Decent or correct in character or behavior.
The word "workaholic" itself has a connotation of being too much focused on work without taking care of other things such as family issues and its meaning is negative. By putting those adjectives like "respectable" or "estimable", you could achieve what you want.
I don't think adding "tireless" before it reduces the negative connotation and it might work in the opposite way.
"he's a real eager beaver."
eager beaver: an exceptionally zealous person, one who habitually takes on more tasks or works harder than others. For example, Bill is a real eager beaver, always volunteering to stay late. This expression became especially popular during World War II, applied to recruits anxious to impress their commanding officers by such behavior. [First half of 1900s] The American Heritage® Dictionary of Idioms
"he's a real ball of fire (or fireball) on the job."