I'm interested in speed running portal but would want to know in your guys opinion which category is easiest to pick up?

Glitchless is a very simple category that any new runner should be able to grasp without much trouble. I recommend going to the "Guides" section of the Portal page and watching WadaWada's tutorial on how to run the category.

I think the best is to learn Legacy or inbounds because there is a lot's of different strats. You can learn easy strat to begin and upgrade your route when you are better. The problem with glitchless is you can't really upgrade your strats because you'll be able to do "everything" quickly, and after get boring because glitchless doesn't really help you to upgrade your skill.
Xeiz did a really good tuto for inbounds & leagacy too.
And there is a lot's of cool trick in legacy & inbounds, if you don't like them you can pick an other strat. Glitchless limit you.

No category is really that hard to learn, only thing is that out of bounds is slightly harder than the other categories. Learn whichever seems the most interesting to you

I only ran Glitchless so far because I felt lazy learning glitches and I don't really like them anyway. Still I ended up learning lots of tricks and stuff, so little do I know now is : Glitchless doesn't mean it's less work xD

I started out with glitchless, and then branched out to inbounds. Legacy is also something to try if you find glitchless too boring.

True Legacy for the ancient is the best choice.
Glitchless has some shit 16 cube hacks that isn't easy for begginers (watch the UD for more info)
The current oob route has to many complicated mechanics for begginers but is very repetitive, so I guess it could be fine if you can learn few tricks.
Inbounds requires learning a lot of SG routes.
Current Legacy has AAG/LAG so shrug.