With Street Fighter games, if you're not a professional esports person, it's generally best to wait a few versions before jumping in. Ultra Street Fighter IV was hands down the best version of that wave of games, as it included all 44 characters, and a range of modes and features.
The original Street Fighter IV was seen as a revival of a dying genre back in 2009. To entice people back the original roster had almost all of the original Super Street Fighter II fighters, along with a handful of new faces. All of this helped retain a familiar feel, despite the visual changes and new moves that each character could use. On the whole, it resonated with the fighting game community and became the marquee game of Evolution tournaments (or EVO as it's more commonly known) for years.
It was a game that divided people's opinions, and at least a small part of that was due to the ultra combo bar that fills up as you take hits. While filling it entirely boosts the power of the move, most people couldn't be arsed. It did provide a bit of tension, in that you never knew when to expect a comeback, but some (including myself) felt that matches began to feel samey. Focus attacks on the other hand, which take an EX bar to use, stagger an opponent for a short time and open the way for extended combos. I've never heard anyone complain about the focus attacks.
The world has moved on from Street Fighter IV, so aside from specific tournaments not many people play unless they're revisiting the whole series. Coming into it new is always going to be a tough ask, especially if you want to play online because you'll end up facing dedicated players who have played Street Fighter IV for the best part of a decade. But still, I'd say this is probably then the best 3D Street Fighter to play - even if it doesn't get everything right.