For me, it'll always be the Premier League. However, that's not to say I wouldn't be absolutely over the moon when finally get our hands on our coveted inaugural UCL trophy.
I feel I would want us to get the UCL because it is viewed as prestigious and it's obviously something the club wants and, as fans, we'd obviously all want to win everything on offer anyway. Plus for the UCL, I feel like it would be that moment when we can finally shut up all those detractors who point it out whenever things are going our way or if things aren't going their way. No doubt, they'd move onto something else then though, probably oh well you've won it once, that's not twice, or you have no fans, which is a laughable comeback.
However, all that being said, personally, for me it is always the Premier League. Finishing top of the league and being Champions after 38 games after almost a year is just an incredible feeling and one I couldn't even imagine when I was a kid experiencing the exact opposite in the form of relegation! Yes, knockout tournaments (basically, the League Cup, FA Cup and UCL) are fun and I love our FA Cup and League Cup wins and the journey we went through for all of them but it can come down to the luck of the draw in such tournaments and a freak result on a bad night/day could see the best team lose out. However, you need to consistently play well in the Premier League and there is nothing else like it!