We'll my favorite is Colosseum but ill go ahead and do a list like the anon above.

Gambit :
+Lots of R-gatcha tickets from pink boxes.
+Has a good cash dump compared to other events (buying pink keys).
+You can be lazy during the event but being active helps a lot since u can unlock more pink caches.
+pretty easy to get the ex-wild card daily (3 sr) a few times during the event.
-Very hard to get 50k (for the sr) if you don't get a slayer from gatcha.
-XP is terrible for the entire event so its actually hard to level up.
-Temporary items kind of suck for this event since Burgers are sort of wasteful compared to other events.


Elite Guard:
+Lots of rewards for killing eg/seg
+big advantage for being an older player, I have 1.5m base sed.
-Still pretty reliant on getting lucky with e-gatcha
-Terrible XP from the event explore (33% efficiency)
+/- Can be sort of lazy for the first week but have to be really active the second week for SEG kills.


Colosseum:
+Most balanced event between free players and paying
+free players can rank really high (I got 82~ during the last one)
+easy to get all of the rewards(srs)
+everything is good in the colloseum gatcha.
+Can get lots of XP during the event due to shakes (I got like 10 levels during the last one 80->90~)
-People with e-gatcha decks are auto-loss
-Have to play the game every 30 min to not waste focus