3* are generally lvled until 10 for feeders
and 40 for 4*s.
The question of "best" is largely dependent on for who and what purpose, it's just that the above is common practice, making it easier to change hands.
3*s are commonly not traded anymore. Though, some via luna (who the hell wants luna???). It's b/c the 4* feeders have become so cheap / alternatively, the price of TE has gone up so much that you don't even want to buy 5xlv10 3* feeders for 1TE anymore.
lv40 4*s are sold at roughly 0.8~1TE each. So a full set can sell for 4TE ~ 5TE.
lv80 4*s are sold at roughly 1.6~2TE each. So a full set can sell for 8TE ~ 10TE.
It is NOT cost/effort efficient to lvl them to 80. But, simply many of them exist since people have been using the 4*s and need it no longer.