I'll be the first to admit that the quality of this manga has significantly reduced after the first 25 or so volumes. Even though I know it's not the best thing ever, I still like it because I've spent so many years reading it. For me, nostalgia does count for something. At the end of the day, ANY long running shounen will decrease significantly in quality after the beginning volumes. Such is the way of things.
If I had to sum up TRC in two words, I would call it a "delightful trainwreck." CLAMP's artistic style is either a love-it or hate-it affair, and I just so happen to love their style. The later volumes can be quite confusing and have many plot twists, but I happen to like that sort of thing. Read with caution if you're anything less than a devoted CLAMP fan.
Eyeshield 21 is a great series, and perhaps one of the best sports manga ever written. It takes all of the standard sports cliches and somehow makes them fresh and original while giving us the ever-popular coming-of-age story. Even people who don't care for American football have gotten quite a bit of entertainment from this manga.
Chrono Crusade was a monthly series instead of a weekly series. As a result, it has some of the best artwork in a shounen series and an interesting, original plot filled with nuns and demons. This one can especially appeal to a female audience for the whole doomed romance aspect. A highly recommended read.
If you're a manga fan and you haven't heard of Death Note by now, you're severely behind the times. Although many people hail it as the best thing since sliced bread, it still has elements of a standard shounen series - the battles are just fought with words instead of fists. The nearly perfect first half of the series is brought down by a wholly unnecessary second half.
Chobits is quite adorable. Often confused for a shoujo title due to its heavy emphasis on romance, Chobits is (for those who know manga) quite clearly a wish-fulfillment title for teenage guys. It has enough cute moments, however, to appeal to both sexes. This just so happened to be the first manga I ever read.
Full Metal Panic! is sheer fun. All the standard tropes are here: a hyperactive female lead (also known as a "tsundere" type), military equipment, socially awkward males, girls in bathing suits, government conspiracies, and lots of mecha battles (of course). If you're looking for a deep, original story that explains WHY everything happens, you'll have to look elsewhere. If you're looking for a good time, try FMP.