I love flexible systems. In my mind there are maybe three or four groups.
Toolkits are the most involved to get set up, typically they are a collection of rules and mechanics that you can use to build up a game to exactly match your desires. I put Fudge and Cortex Prime in that group.
‘Generic’ systems are a full and immediately playable game with rules to cover a wide range of contexts, usually with optional rules to tweak them to fit your specific need. I prefer to call these ‘genre flexible’ rather than ‘generic’ since that is used as a pejorative in many cases and creates a false impression of homogeneity. More on that later. Savage Worlds and GURPS are good examples here.
Those two are the most clear-cut categories in my mind.
Somewhere between a toolkit and a genre-flexible system is what I tentatively think of as a core system. These have more pre-construction than a toolkit but less than a genre-flexible system. Fate is a good example in that it is more pre-assembled than Fudge but not as much as GURPS is.
The last group are house systems. These are systems which are used by a publisher for multiple different games but where there is no published toolkit or ‘generic’ version. 2d20 is a good example from recent game lines but there have been many of these. There is an SRD for 2d20 IIRC correctly. The d20 system might also be seen in this way given how much it is used for different RPGs.
All of the systems I have read / run that fit into this space have a different feel to them, and come from a different perspective or baseline. That is why I dislike the name ‘generic’ - that suggests something without its own identity and experience says that is far from the case. A game run using GURPS will be different to one run with Savage Worlds or Fate, even if they all address the same basic concept.
The question of where best to start kind of depends on a few things in my opinion.
Understanding the feel you are looking for, and finding the base systems that most closely matches that, is useful. For example, HERO was originally used with superheroes and is a great fit for games at that level of character ability, while GURPS aims to simulate the ‘real world’ which makes it a more natural fit for games that are grounded. Savage Worlds aims for action-adventure so is somewhere in between.
Second, the level of granularity you want out of a system is also something to consider. GURPS and HERO are both more granular than Savage Worlds or Fate for example.
Third, what do you envisage characters likely doing or games involving? Different systems have good mechanisms for different kinds of things built in to them, or provide the tools to make such things. Cortex Prime has a good implementation for handling character relationships (including antagonistic relationships you might see in a recurring drama with rivals, for example).
Then there is the question of worked examples. Systems with lots of support tend to be easier to learn and get your head around, at least I have found that to be the case. Some games have lots of explicit GM guidance on how to use them, other have lots of worked examples, having both is obviously best.
All those factors will influence which system a particular GM might find it easiest to start with. Using a system that is closer to your desired outcome will mean less work is needed. In some cases, using a flexible system for a campaign far from its baseline can be done but will feel clunky. I think this is where a lot of negative experiences arise from as people end up feeling like they are fighting the system
