I’ve had investment accounts with Hargreaves Lansdown, Interactive Investor, iWeb, Freetrade, Dozens, and a couple of Roboadvisors (I forget which ones). Of these, I would say Hargreaves Lansdown has the best app.
However, I don’t think best app should be particularly high on list of priorities when it comes to investing. I currently use Freetrade, primarily due to fees.
If I was just starting out and looking to open an S&S ISA, I’d probably go with Vanguard Investor (cheaper than Freetrade for ISA when starting out). However, it doesn’t have an app.
The following thread is worth a read