For GT-i9300, I highly recommend installing Archi Kernel, which will give you great battery life (as the kernel is built on Samsung sources) along with overclocking up to 1.6Ghz. (from default 1.4Ghz).
Upon installing LiquidSmooth ROM, you will find a nicely-themed Liquid Launcher, which is slightly modded KitKat launcher with support for modifying your grid size. If using a phone without hardware keys, you will also find nice Android L-styled soft buttons, which also you can customize and add up to 5 different buttons.
For Galaxy phones or phones with AMOLED screens, make sure you enable “Total blackout” option in Settings as that will save you some battery. AMOLED screens use zero energy on black pixels so having this on will save you a bunch of battery life.
For customization, LiquidSmooth ROM offers one of the best customization options through its LiquidSmooth settings (in Settings). You will be able to customize everything from your lockscreen, status bar, navigation bar, CPU, and much more.
Navigation custom setttings are my favorite part of the ROM, you have many choices including App circle sidebar, Gesture anywhere (shortcuts using gestures), PIE Controls (two PIE controls on-board), and customization offered for your soft buttons plus navigation rings. By using your navigation options carefully, you will be able to maximize efficiency of your phone while on the go.
For notifications, LiquidSmooth ROM offers Active Display, Driving Mode, HALO, Heads Up notifications, lockscreen nofitications, and some more.
Many of the LiquidSmooth builds also come with an overclockable kernel, meaning you will be able to easily overclock for your high-end games/graphic-intensive apps.
If you are looking for a good Android 4.4.4 custom ROM with lots of tweaks/customization, definitely head up the latest version of LiquidSmooth ROM v3.2, it is now available for all Galaxy S3 variants including AT&T, T-Mobile, Sprint, Verizon, GT-i9300, GT-i9305, and Canadian.