Some of my fave Genny shmups:
ThunderForce IV
Raiden Trad
Fire Shark
Sol-Deace
Gynoug
M.U.S.H.A.
Twin Cobra
...etc...
Gleylancer is slightly overrated IMO, considering its high price tag. It plays R-Type-ish with some Darius/Gradius elements thrown in (like the sidepod system) for good measure, graphics and sound are very good, but well, I'd recommend anyone who plans on buying this one to try the rom first. Apart from excellent presentation and lots of story bits with partly animated sequences, the game pretty much is standard horizontal shmup fare.
If you want the most intense horizontal shmup action, there's no way around ThunderForce IV... I LOVE this game, every little bit of it.
And yeah, Gaiares is great as well.