Which episodes of Flash cross over with Arrow?

Arrow and The Flash are part of same canon called Arrowverse and have crossover almost every year and even more then once sometime:

• The Flash S01E01: It's Pilot for the Flash show and Arrow came for a speech/dialogue only and don't have much impact of it on Arrow show itself, You might already know S02E8 and S02E09 of Arrow is kind of backdoor pilot of Flash and connect to S01E01 of Flash.

• Flash vs. Arrow - Flash S01E08, Flash also return the favor in Arrow S03E08 but both episode can be watched separately.

• Heroes Join Forces- Flash S02E08/Arrow S04E08, it's a full two epsiode arc and need to be watched together in telecast order (First Flash then Arrow) and it serve as Backdoor pilot for Legends of Tomorrow.

• Invasion! aka Heroes vs Aliens - It's a three-way crossover between The Flash, Arrow and Legends of tomorrow with additional few minutes in the near end of Supergirl S02E08 (which can be skipped). But it's a full single arc and need to be watched in certain order , first Supergirl S0E08 ending then The Flash S03E08 then Arrow S05E08 and ends with Legends of Tomorrow S02E07.

• Crisis on Earth-X- It's fourway crossover between Supergirl, Arrow, The Flash, and Legends of Tomorrow or fiveway if you count Freedom Fighters: The Ray. It's about a new earth called Earth X and all the major characters from the shows appear in it.

Flash and Arrow both have Cameo in Vixen Season 1, whole season one act as single episode and can be watched in CW seeds. Same happen in season 2.

In web animated show Freedom Fighters: The Ray, we meet alternate version of the character from Flash, Arrow and Supergirl and also Cisco had significant scenes.

Similarly Arrow have cameo in Legends of Tomorrow S02E01 too.

Other smaller crossovers:

• The Flash S01E04, Felicity visit Barry and help them to deal with there meta-human issue of that episode.

• The Flash S01E18, Felicity and Ray Palmer visit Flash and team regarding help for his A.T.O.M. suit.

• The Flash S01E19, Cisco Ramon and Joe West visits Starling City to get help of Quentin Lance. Cisco meets Laurel Lance and upgrades her sonic device.

• The Flash S01E22, Team Barry try to move the meta-humans from the Pipeline at S.T.A.R. Labs to the A.R.G.U.S. prison on Lian Yu but Snart double-crosses the Flash and frees them. Barry then fights the Reverse Flash, defeating him with the help of Oliver Queen and Firestorm.

• The Flash S02E15, John and Lyla visit Star City and help Barry defeat King Shark.

• The Flash S03E02, Barry goes to Star City to talk with Felicity about what mess he did, it did impacted Arrow show as:

  Diggle has a son now rather than a daughter.