Symphony No 1: Shostakovitch Symphony No. 1
Shostakovich's first symphony is one of the best first symphonies ever written. While not truly representative of Shostakovich's style as it was his graduation piece from Maximilian Steinberg's composition class. There is some obvious Stravinsky influence in the work as well due to his (at the time) recent exposure to the composer.
Runners up: Mahler 1, Boulez Symphony Fantastique
Symphony No 2: Rachmaninoff
Rach 2 is just an amazing symphony. Reminiscent of his 2nd piano concerto, it is a beautiful work with a lovely clarinet solo in the 3rd movement. Not too much to add, I'm sure there's a nice backstory but I'm not gonna look for it right now.
Symphony No. 3: William Schuman
Why this instead of Brahms 3, one of the most beautiful pieces ever written? It's because I really like this piece and it doesn't get enough exposure!! It's a really unique and has a pretty epic bass clarinet solo. The only real recording of it is with Bernstein, though I believe Seattle has a pretty good recording of it as well.
Runner-ups: Brahms 3, Saint-Saens 'Organ' Symphony
Symphony No. 4: Tchaikovsky
Tchaik 4 over Brahms 4 is an easy choice. This piece is pretty awesome and exemplified the anti-nationalist themes Russian composers hid in their music.
Runners up: Brahms 4
Symphony No. 5: Prokofiev
The fifth symphony has the toughest contention. However, I chose what I liked most rather than what has the most impact or most historical value. Therefore, I picked Prokofiev simply because I love this symphony. It's so comical but also so sad. Each movement represents a certain facet of Russian life, but also Prokofiev's own reflection of Russain society. It is both endearing and sarcastic at the same time.
Runners up: Mahler 5, Nielsen 5, Shostakovitch 5, Beethoven 5
Symphony No 6: Beethoven
I think 6 is better than 5! And on that subject we have Beethoven's 6th symphony! Also known as "pastoral" symphony it is a great demonstration of setting scene and ideas in music.
Runners up: Tchaik 6 (Pathetique)
Symphony No 7: Dvorak (aka 2nd symphony)
The 7th symphony is my favorite of Dvorak. The slow movement sounds like Puff the Magic Dragon...
Symphony No 8: Schubert (Unfinishied)
Just a great piece of music... sorry just want to get this done.
Symphony No 9: Mahler
Sorry kinda a low-quality post... but they're great pieces trust me!