Digimon World DS

which one digiolves into the better mega gen Greymon or GeoGreyMon?

Greymon can become either WarGreymon or BlackWarGreymon. Can also become Omnimon, but you need to already own an Omnimon via DNA Digivolve to be able to digivolve to it normally.Unfortunately, GeoGreymon doesn't have a Mega level in World DS. Only in Dusk/Dawn.

ok now i need help on how to beat skullgreymon

There is a guide on GameFaqs you know.....http://www.gamefaqs.com/ds/928689-digimon-world-ds/faqs/46899

checked that before post
and i have all ultimate equips but none of my digimon are high enough to equip

Then you should probably grind a bit more before you face SkullGreymon.I haven't played this game in a while so I can't remember much on boss battles.

btw skull grey mon probs has like 500-2000 hp