と言(い)うのは事実(じじつ)だIt is a fact that ~, There is no doubt that

Both phrases assert something strongly, but from different angles. ～というのは事実だ affirms something as a literal fact, e.g. 負けたというのは事実だ ('It's a fact that we lost'). 何といっても expresses what the speaker feels is most important, e.g. 何といっても家族が大事だ ('No matter what you say, family is important').

Both ～というのは事実だ and に違いない make confident statements, but differ in implied certainty. ～というのは事実だ affirms something as a fact, e.g. 負けたというのは事実だ ('It's a fact that we lost'). に違いない expresses strong conviction, but not absolute certainty, e.g. 彼は疲れているに違いない ('He must be tired').

Both emphasize a truth, but differ in nuance and tone. ～というのは事実だ explicitly affirms something as a fact, e.g. 彼がやったというのは事実だ ('It's a fact that he did it'). もさることながら highlights one point while further stressing another, e.g. 見た目もさることながら味も最高だ ('Not only the appearance, but the taste is excellent'). Both are formal, but the former is more clinical, while the latter sounds more literary or poetic.

These two forms both relate in a sense to an unchangeable fact, but differ greatly in usage. ～というのは事実だ literally states that something is a fact, e.g. 何もできないのは事実だ ('It's a fact we can't do anything'). どうせ expresses resignation or futility with a more emotional nuance, e.g. どうせ無理だよ ('It's hopeless anyway').