Full suggestion:
Charizard (helps vs 4th 7th and 8th gym), helps vs lorelei e4 (just a bit tho, she has water and ice types), bruno e4 (if you have flying attacks), champion.
Gyarados (helps vs 6th gym with crunch, 7th with waterfall, 8th with equake) , helps vs agatha e4 (crunch), lance (any dragon move after dragon dancing), champion.
Gengar (the GOD mvp), helps vs 4th gym (using poison moves), 5th (psychic moves), 6th (shadow ball), 8th (shadow ball), lorelei (use thunderbolt), bruno (use psychic), agatha (use shadow ball), lance (icy wind/thunderbolt), mix moves vs champion.
Personal suggestions:
Machamp destroys 5th gym with equake (ask someone to sell you the tm), helps in 7th, 8th (dangerous though), destroys lorelei, helps vs bruno, helps vs lance (if it has rock slide), is useful vs champion.
Alakazam (Gengar's best buddy), destroys 4th gym, 5th, 6th, and helps vs the 8th one. In e4, it destroys bruno and agatha, it can somewhat help vs the others.
As a last, Magnezone: almost immune to 4th gym, 5th gym immunity, 6th gym resistance, 8th gym can tank, destroys lorelei, helps vs agatha, destroys lance, helps vs champion.
Good luck!