“Best chance” of unseating him. Beto was helped by boatloads of money and an extremely disliked opponent (even among Republicans). No way either of them do better in 2020.
i think i probably agree. but to devil's advocate - castro v cornyn would probably bring out the latino vote more heavily dem than beto v cruz did. and if you could count on the presidential election to turnout the black and anti-trump white vote, you could get a more favorable result.
also, cornyn has a lower net favorability than cruz. people don't hate him as much, but they also don't like him as much. so i'm not sure how that translates into votes.
finally, texas gets, probably, something like about 0.5 % more dem every year, demographically. so it should be about a point less of an uphill climb in 2020 than 2018.
all that said, if beto thinks he could win the nomination, beating trump would be a lot easier than beating cornyn. the hard part is getting nominated.