you should state what version of android os you are running.
for 2.2 (froyo) and 2.3 (gingerbread) you DO NOT want a a task killer as it will drain your battery more (the apps continually restart up).
the battery minders that allow you to kill data usage (you can set specific sync times for programs instead of auto) will save you a little bit of battery but can be more hassle then they are worth. for the record i stopped using one long ago.
you gain the most battery savings from turning off wifi, gps and bluetooth when not being utilized. turning down the backlight to 15% or so is good as well. some say using dark backgrounds helps them but i never noticed a real difference.
as far as an AV i'm not sure. if you are going on alot of questionable websites or downloading/installing questionable content then you might want to look into it more. i don't think the threat is at a point yet for an AV to be mandatory.
if you want to change the whole look and feel of your homescreen while remaining stock then look into what is known as a launcher. the best imo is "go launcher ex" but the other two popular ones are "adw launcher" and "launcher pro".
the robotic tts voice for nav blows. download a program called "svox classic" and you can preview the voices they have for sale. i personally use "victoria, uk english" for that classy brit sound.
if you dislike the stock sms app, try "chomp sms"
if you dislike the stock music player, look at "power amp"
it is definitely wise to get "titanium backup" and to backup your phone once in awhile just in case.
if you want to be able to click on one icon to open up a folder full of organized apps (ie tools, media, email apps...) then look into "folder organizer"
if you want to create clickable areas on your desk which can be targeted and customized with an image of your choice look into "desktop visualizer"
if you like minimalist styles (such as weather, date, time displayed in text not icons/numbers/etcetera) then look into "Minimalistic text"
i'm currently using a nifty little clock widget called "advanced clock widget". not a whole lot of features but it fits some home screen setups.
if you want to place custom icon bars/toggles/controls in your notification area or on your desk then look into "widgetsoid". i used to use this to place the wifi, gps, bt on/off controls and backlight % control in my notification area before i rooted.
if you dislike the stock keyboard you might want to try "swype" or "swiftkey". personally i dont like either of those two and love "hackers keyboard" which is almost an exact replica of real computer keyboard.
gmail, gmaps, gnav, gtranslate are all must have apps. gskymaps and gearth are okay if you're into that sort of thing. voice search works okay but only if you speak VERY clearly. i find it faster to type.
as for what i personally use... i rooted a few months ago and am running the MIUI rom which was translated from what was developed in china. free (yes that is right, FREE, and WORKING) wifi tether for your laptop is stock in the rom. power controls are built right into a two part notification screen.
MIUI comes standard with no app drawer so i run go launcher ex instead of the stock launcher. this gives me the customization i want on my desk as well as an app drawer. all of the rest is 100% MIUI.
if i finish creating my homescreen (for literally the 60th time) tonight i'll try and upload a photo so you can see my minimalist style i'm using.
i would like to create a theme for MIUI as well if i can figure it out but i doubt that is in the near future.
i'll end this post with a warning: beware.... you'll be changing your home screen, icons, and apps all of the time! "stylizing" your phone can become addictive.