My top ten,
Mahler
Sibelius
Vaughan Williams
Martinu
Shostakovich
Prokofiev
Nielsen
Holmboe
Kokkonen
Pettersson
Honorable mention: 13 other 20th century composers whose symphonies I listen to from time to time,
Persichetti
Valen
Englund
Piston
Ropartz
Tubin
Melartin
Roussel
Knussen (although Knussen's one movement 1st Symphony was later withdrawn, and revised as his Concerto for Orchestra, so he may not count?).
Norhølm
Rautavaara
Panufnik
Arnold
I listen to Magnard's 4 Symphonies, too, but he doesn't count, as CnCB pointed out, since Magnard only composed his Symphony no. 4 in the 20th century.
(I also occasionally listen to the symphonies of Honegger, Harris, Sessions, Henze, Lutoslawski, Atterberg, Bax, Schuman, Rubbra, Simpson, Rochberg, Heininen, Hanson, Rorem, Dutilleux, Vuori, Pärt, Penderecki, Maxwell Davies, Tippett, & Nørgård...)
Charles Koechlin would be on my list, as well, but his symphonies remain unrecorded, except for "The Seven Stars' Symphony", and a live BBC radio broadcast by Konstantin Silvestri of his war time Symphony no. 2:
Langgaard's Symphonies are still on my list to explore, although I've heard & liked his "Music for the Spheres" (but have yet to connect to one of his symphonies--any suggestions?). I'd also like to hear the symphonies of Aarre Merikanto, Christopher Rouse, and maybe Poul Ruders & Phillip Glass...
I've not explored the symphonies of Alfred Schnittke, either.
Among present day composers, I'd like to see Magnus Lindberg and Anders Hillborg compose symphonies...