Depends on what you want. I mean, you can just pick up the movie cast, or get them all. It all depends on what you like/want.
I just got the ones from the movie, didn't make sense to me to get ones that weren't even in it. The Voyager line and deluxe is cool. I stayed away from the leader ones.
Depends on the age of the recipient. If they are under 5, get cyber slammers. If they are under 8, get fast action battlers. Then move up to legends class, deluxe, voyager, etc. I know that there are a lot of kid geniuses out there but most 5 year olds (and many adults) cannot transform a voyager class figure, despite that 5 and up recommendation. Basically you have to check the age and skill level of the recipient. I do not recommend deluxe figures for everyone. Besides, if you want to buy the main movie characters, good luck finding them. Real Gears are always overlooked but well worth the value. Except for that digital camera, all of them are easy to transform and look cool. You will always find Real Gears.
Toys That Are Undoubtedly Good: Dlx: Dropkick, Wreckage, Longarm, 08 Bumblebee (which everyone else loves but me... it's a meh toy imo), Barricade, and Landmine Voyager: Optimus, Blackout, Ironhide Toys that I think are good but others aren't keen on: Dlx: Arcee, Dreadwing, Swindle (In my opinion these are three of the best in the movie line, but not many others seem to like them), Stockade. Ones to avoid: Scorponok, Brawl (I like it but it falls apart way too easy... and that arm problem... shudder), 74 Bumblebee and Payload (see Brawl)
Leader class Brawl is a wonderful TF. Looks mean as a display piece, bears a good likeness to his movie self, is insanely loaded out with weapons (two of which have automorphs), and has hardly any kibble apart from the back of the legs. Bit of a challenge to transform but that just adds to the fun, for me anyhow. Just wish they'd painted his shoulder-rocket tubes black but hey, ce la vie. I also put forward 08 Bumblebee, Ironhide, Landmine and Longarm for serious consideration. But I don't have many movie figs so my personal experience of the line as a whole is sketchy. Stay away from Deluxe Brawl. As has been mentioned, the arms are faulty and won't peg in.