It's always a tiefling offspring.
The PHB, in the section on tieflings says (emphasis mine),
Their appearance and their nature are not their fault but the result of an ancient sin, for which they and their children and their children's children will always be held accountable.
Based on that, I would say a tiefling/human pairing would result in a tiefling. The offspring may have less fiendish features than your average tiefling, though that would be a character choice one would make/discuss with the DM.
Taking it a step further...
If we make the assumption that since tieflings can interbreed with humans, they can interbreed with the other races that humans tend to interbreed with - e.g. elves and orcs - they would still be in the bloodline of the original pact. So a hypothetical elf/tiefling pairing would yield tiefling offspring. Though, again, the player and the DM could have some fun with that in terms of game mechanics. Maybe that elf/tiefling (elfling?) has the Fey Ancestry trait in place of one of the tiefling racial traits.