"Halo is the superior series."
Depends on what you want. Halos core gameplay mechanic of destroying a mixed group of Covenant with Elite a couple of small guys etc. is the most fun of any FPS. I liked the repetitive indoor levels of HAlo1 because it was so much fun to kill these guys. Also Halo has awesome music which is often undervalued.
HAlf-Life2 created an awesome depressing world and is more grown-up. I dont know which is better. Both are great games but very different.