I've kept this over for wayyyy too long. I figured that, since I wrote a paper on him last semester, I would easily write a summary about why Senator Flake has become so popular. I'm just going to include parts of my paper that will help you understand why he's many people's (mostly Democrats and Republicans who don't like Trump) favorite politician.
On October 24, 2017, Republican Senator Jeff Flake of Arizona declared on the Senate floor that he would not seek reelection. In his speech, he denounced the Republican President Donald J. Trump and the actions of many of his Republican counterparts, contending that they were “complicit in undermining [American] values” (Flake 2017). Many citizens—Republicans and Democrats alike—were left in shock at his words and decision. [Various political theories I doubt anyone cares about, Republicans support Trump now though they disliked him during the campaign because they won't get reelected if they don't support him and his policies.]
Yet while most Republicans fell into line to support President Trump, Senator Flake did not. While Senator Flake continued to vote for Republican policies, he spoke out against President Trump, even writing Conscience of a Conservative: A Rejection of Destructive Politics and a Return to Principle, a book which included his condemnations of the president and the new direction of the Republican Party (Graham 2017). As one would expect, Senator Flake soon began to experience backlash from his party. President Trump took to twitter to attack Senator Flake, even supporting a Republican candidate who planned to challenge Senator Flake in the upcoming election (Stracqualursi and Wells 2017). [In addition, Flake lost A LOT of constituent support and Bannon predicted Flake would lose.]
[Everyone predicted Flake would stand down and support Trump so that he could get reelected. But surprise, surprise-] Senator Flake, instead of supporting President Trump, announced his retirement from the Senate. During his speech, Senator Flake argued that there were “times when we must risk our careers in favor of our principles. … A political career doesn't mean much if we are complicit in undermining those values” (Flake 2017).
The senator’s speech not only contained his announcement of his retirement, but also an attack upon President Trump. ... Senator Flake addressed Republican legislators and voters, asking for their “complicity and…accommodation of the unacceptable to end” (Flake 2017). Senator Flake based his retirement and speech on his principles and his dedication to the American government the founders had envisioned. It may be contended that, if Senator Flake truly intended to fight for his principles, he would have elected to remain in office and try for another term than to desert the institution he loves. However, Senator Flake knew that he had little chance of winning the upcoming election, and his words would carry more weight if spoken by a seated senator instead of by a politician who had just been voted out of office.
Basically, Democrats and Republicans who don't like Trump think Flake is great because he refuses to compromise his values and principles, even at the expense of his own political office. Others point out that Flake's campaign was already failing, so his speech was wasn't inspired by his principles and more by his certain future electoral failure and desire to get attention. (I mean, I haven't checked, but I bet his book sales skyrocketed after his speech.)
Also, Flake is a Mormon, which you can totally tell if you listen to his speech. I could of sworn I was watching General Conference. The "General Conference authority voice" is definitely a thing.
In addition to all this, Flake has a different view of immigration than most Republicans, as he supports immigration and a bipartisan DACA. This has definitely affected his support levels among Democrats and Republicans. While he certainly isn't everyone's favorite politician (*cough* my dad *cough*), he definitely is more well-known and popular than he ever has been before.
tl;dr - Flake is a soon-to-be-retired Republican Senator who denounced President Trump and likes immigration.
-guppy of doom