To say this is simply a matter of opinion is the understatement of the century. To each his own, everyone.
i personally think GF III is much better than Goodfellas. I enjoyed Goodfellas, thoroughly, but i despised the ending sequence as i found it quite dreadful. this is where i always have problems with Scorcese- in all of his movies he has some 'thing' that some people will either love or hate. i find that to be quite a boring way of encourageing dialog about one's film.
GFIII was a completely different movie than GFEllas because it was more psychological. it might have been less 'fun' to watch but not less interesting at all.
In regards to whomever made the comment of Pacino not being in it, i think you have something there but not on a literal level. they didn't have A
'pacino' because ray liotta is no al pacino. sure Pesci won an Oscar, but it wasn't a historical character by any stretch of the imagination nor were Deniros or Liottas.
Just my two cents...