In my personal opinion it's what you play I am a woodsballer so slider shorts were first then elbow/forearm pads and honestly at the fields I play at its mostly standing and very little kneeling so knee pads aren't important.
Look at what is feeling sore the most after you play, if you find your knees are bruised/sore after the day go got a pair of knee pads, your arms and elbows bruised grab those, I found slider shorts just overall nice since my pants are lacking in protection in that area (not just the jewels but all around) but my knees have a good amount of protection so I found knee pads not necessary
Well I usually get tagged while on the run when I'm playing woodsball because I'm usually running around like an idiot, and the last time I played it was a whole day of woodsball, and I had welts everywhere (was at a field where blaze is the only option, and that shit gets you bad when it bounces sometimes.) but the especially bad blood leaking ones were all over my legs. Maybe pants? dont have pants yet. that kinda seems like a good idea now but I'm about to purchase a new pack, so im only gunna have like 55 to spend on whatever the first set of pads I get.