I love Fairy Tail and its humour + fanservice; it's ultra unique and hilarious! I'm not usually watching OVAs of the anime, usually I watch if it's worth it and THIS is REALLY worth! :D I didn't knew in which order to watch nor in which episode they are between T^T So I carefully picked and now decided to give you advice how to watch them :D
FT- OVA 1 ~ Welcome to Fairy Tail Hills!
This OVA take places around episode 68- after the battle with Oracion Seis.
Lucy finded a job request on guild' request board which have no reward and went to see what the job invloved. Mirajane asks Lucy to investigate the mysterious job and she find out that is place in Fairy Tail Girl Dormitory - Fairy Hills. The client was an old woman called Hilda who introduced herself as the owner of dormitory. She asked Lucy to find a treasure that she've lost but do it in secret so dormitory girls won't know. In addition she told her to wear the sexy cat-costume and saying meow after each sentence. On the other side the boys were asked to clean Fairy Tail swimming pool and founded out a peeping whole. So what and where is the treasure of Fairy Hills? What's with the peeping hole in the pool?
FT- OVA 2 ~ Fairy Academy
If you watched to the episode 65 or so you should be fine
It's a parody of everything that happened so far but in normal life and academy edition. The funniest thing is that there's no magic and they just experience normal yet weird life. Lucy is a transfer student who started, not a long ago, to attend the Fairy Academy - school full of weirdos. Lucy's Celestial spirits are teachers as well as the cats and everybody else are students (including Makrov). Erza is the class rep and asks Wendy and Lucy nervously for help as she has a date with a boy form Era Academy - Seig. Also there's a problem between the Courage Academy (which is made of Phantom Lord's members).
FT - OVA 3 ~ Memory Days
While Lucy, Erza, Natsu, Happy and Gray were cleaning the storage room, a strange book fell down which sent them into the past. The guild's bulding is back to its original design and Erza, Gray and Natsu are kids who started a fight. The 'older' Natsu and Gray enjoyed the scene while Erza was explaining that they shouldn't make contact with anybody as it may have horrible consequences to the future. Before she had finished explaining they simply disappeared. Erza then tells Lucy to find them while wearing the sexy bunny costume so they would make no contact with anybody (i guess Erza just caught a chance to dress up cutely with Lucy XD). They need to go to their present time as fast as they could.
First I thought the OVA was most about telling character's past and getting out of the trap yet that time-travel was important. Suprisely, there's a plot twist XD + Erza's funny moments + Lucy's important memory. Afterall, this Ova wasn't that bad......
FT - OVA 4 ~ Training Camp
This OVA is at episode 153.
To train for Grand Magic games, the Tenrou Team and the rest of Fairy Tail members split up into groups and went to different places to enhance their magic. Erza, Natsu, Gray, Jet and Droy, Levy, Wendy, Lucy and Juvia went to a beach to train (don't ask why and how would you train youself on a beach XD). Other teams went somewhere else; Mirajane's team went to the mountains, Raijinshuu somwhere far away, and Lily and Gajeel have kept it in a secret. Erza's team decided to have good time on the first day of training while Mavis who was bored in Tenrou island was secretly watching them. As this ep takes place on the beach there's lots of fanservice. In addition, which is my fav part, is when boys entered the room and saw all the girls drunk. Later the girl went to hot springs and where talking about who is in love with who :D Y'know just a girly talk.....
FT ~ OVA 5 - The Exciting Ryuzetsu Land
I think this OVA is between the Grand Magic Games - so if you had finished the first season (or about to) you shoul be fine.
The guild members decided to go and relax in one of the popular places in Fiore - Ryuzetsu land - which is a big water park. So much Fanservice now! Everybody changed into their swimming suits except Cana as she decided to wear her underwear. Then we Gajeel, Levy and the three cats looking for attraction, Lucy and Erza wandering around, Elfam and Evergreen ashemed and hiding behind the rocks (a date? ♥), the love trio - Gray x Juvia x Lyon and other. Is just like a bunch of humour and don't want to spoil but I think it's the best OVA.