I think you'll have some trouble finding definitive answers for this, unfortunately. There is a ton of flute repertoire out there, and some people like certain things while others like other pieces. I'm a fan of Romantic era and modern music, but many people dislike modern. There are pieces that virtually everyone plays, whether they like it or not, such as the Chaminade Concertino, but you'll have a tough time finding a consensus on pieces that any flutist would want to play.
The Haendel Hellenser sonates (+ Basso continuo), though I have read they are sometimes considered to be written for other instruments (such as recorder and in any case not for the modern flute, obviously).
They are relatively "easy" to play for someone with 3-4 years regular study (IMHO)
the answer may lie, as it does for much music, in the ability for a piece to last, to remain a classic.
When I was in music school, we were told to learn avante-garde pieces as they would be all we would be playing in our careers. The opposite has come true, all those pieces I had to learn many "ears" ago have gone by the wayside and I play what I know the audience will like.
...I've been looking in my score library...you also have other pieces
like :The Butterfly from Kohler (a nice concert etude by the way ),Gaubert's sicilienne (level 5 or 6 ), Czardas from Monti,5th Concerto from Vivaldi obviously, Les Joueurs de Flute from Roussel(all of the four pieces "pan" "krishna" "tytire" and "mr de la Péjaudie", Concerto in G major from Pergolesi,I think that Bach's Badinerie have already been mentionned (but you have all the other pieces of the orchestral suite in B minor), the wonderful (and not that hard) Reverie from Caplet, Devienne's sonata (havn't studied it yet), Doppler's "Airs valaques",and those I 'm working on now, Pan from Jules Mouquet (all the the three pieces are great to play,"Pan and the shepherds", "pan and the virds ", "pan and the nymphes"...there you go !
The original cult classic theme to Mission: IMPOSSIBLE. Composer/Pianist/Conductor Lalo Schifrin got a special composer's award from BMI in 2001 for that work. It is certainly popular and written for flute, so does it qualify? I vote YES!
I have no idea if these pieces were for another instrument or whatever but I really really enjoy Teleman flute peices. I like the Fantaisia book for flute (I almost spelled flute floot here oh my), Suite in A minor, and there was a book I had with 4 sonatas of his and of those my favorite is Sonata in C minor then F major. I also like many Bach pecies especially Sonata in A minor without accompainiment, and another of his book collections had nice pecies 6 sonatas for flute and piano especially sonate 1 first mov, Sonata 2 (the siciliano spot is slow and very pretty)and sonata 3 very pretty. Also I belive the composer is Aurthur Honnager (I probably buthered the spelling of that sorry)Danse De La Chevre which I thought was a goat was a pretty differnt peice and enjoyed it very much. That was for flute alone too I think. Syrnx I have not played but is absolutely gorgeous by Debussy. There's also another book called 24 Short Concert Pieces revised by Cavally that have several nice pieces. I really liked in that one Scherzino by Anderson is very fun and has some pieces by Quantz who is nice as well.
I tend to like lots of baroque stuff. Lots of moving lines and just like the way those pieces sound.