The best poker Solver?

Best poker solver is probably private

I am very happy with the latest PIO (3.0).

It depends on what you are playing and what you want to do.

The best preflop solver right now is HRC. It will require a pretty decent computer to run or to rent servers for heavy work. It's a no brainer if you play MTT, interesting if you play spins, marginal if you play cash (haven't done a lot of CG work on it so maybe I'm wrong). It can also do multiway work reliably unlike other older competitiors.

For postflop it's more unclear, but for a beginner in solver PIO has a huge market share so you'll benefit of the advices of the community. If you are on macOS you'll have to run it on Parallel since it's a Windows app. PIO can solve any HU situation in holdem and has some complex analysis builtin tools.
An old PIO2 licence will do as well as a subscription to PIO3.

GTO+ is the best value for money. Plus you get a free gto trainer

I've migrated from Pio (old license) and very happy so far

What do you need from it? For a lot of purposes you can just use the solver in GTO Wizard Elite subscription. It is super fast, much much faster than what PIO would be even on very expensive server. You cannot run it down to a very high accuracy and you cant use it for things like aggregate reports, but it is enough for most casual usage. I coach people for a living, including highstakes pros and I use it all the time, almost forgetting I even have PIO. (which obviously can do more things, especially nodelocking options are more advanced, but you dont need that so often).