I've had the best luck with Hasagawa and Tamiya, but these are high-end kits and cost a bit of change. If you're just starting out and want to get some good kits on a budget (it sucks to ruin a $20 kit), take a look at the Revell/Monogram line and the Heller and Airfix lines. All 3 companies have been around a long time and can put out some good stuff (stay away from the Snap-Tite and Skill Level 1 kits as these tend to be too easy to assemble and a bit short on detailing).
Right now
http://www.modelexpo-online.com has their Airfix and Heller kits on half-price sale. I picked up a 1/48 Airfix F4U-1A Corsair and 1/48 Heller F4U-4 (AU-1 official designation) for less than $7 each.
I source most of my kits from
http://www.modelexpo-online.com and if there's something I'm looking for that they don't have, then I go with
http://www.ehobbies.com or
http://www.internethobbies.com. I've had good experiences with all 3 stores.
For 1/72 scale, Hasagawa is the king. For value, the 1/72 Heller P-38J, P-40 and Me-109K kits are good, very detailed kits (considering the money) and most of the 1/72 Airfix line is good, though they do produce a couple of Skill Level 1 Spitfire and 109 kits that are ok, but their higher level kits are better for only a dollar or two more.
A couple of specific kits I like are:
Revell of Germany U-47 Gunther Prien (RG5060)
1/48 Airfix F4U-1A (No. AX5106)-This is the British version, without the birdcage canopy. You have the option of shortening the wings for the RAA version, or keeping the wings normal for the RNZAF version
1/72 Airfix P-51D (No. AX2098)
Heller Le Suroit Research Ship (HEL80615)-N-scale railroad figures really add some life to this kit
1/48 Heller Corsair AU-1 (HEL 80418)
1/72 Heller FW-190A8/F3 (HEL80235)
1/72 Heller Corsair F4U-1 (HEL80275)
1/72 Heller BF109K-4 (HEL80229)
1/72 Heller F6F-5 (HEL80272)
1/72 Heller P-38 (HEL80273) - molded in silvery plastic so its good for weathering and simulating battle damage if you want to experiment. Heck, for that price get 2 and build one straight up and one battle-damaged
1/48 Revell/Monogram Tiger Shark P-40B (RM5209) - the tooth decals are a pain to do, but they look good with patience.
1/72 Revell/Mongram SR-71A Blackbird (RM5810)
1/48 Revel/Monogram FW190A8 (RM5943)
Really, any of the 1/48 Revell/Monogram kits are a good value, including the new re-releases with the old fifties-style boxes.
Avoid the Heller Dauntless kit - I had some bad wing warping with my sample and the details really weren't up to par. Also avoid the Heller P-51 kit as it tends to lack details and the seams are way off.
That should get you started.
And if you want to do some quick car kits, the AMT/Ertl pre-painted kits (not the snap-together ones) are a lot of fun and look really good.