So, there are a lot of ways to challenge PC's.
Let's start with a standard humanoid force package.
Humanoids can have real problems against PC's and are generally worth less than their CR especially in numbers and especially when the difference between the CR and the player level gets to be 4 or more. Take an example like a Gnoll. At 1st or 2nd level, this is reasonable challenge. But eventually the Gnoll's low BAB makes it virtually useless. Those humanoid HD are wasted. In general, to compensate for this, I tend to avoid humanoid HD and 'warrior' classed humanoids. Humanoids in my game tend to have zero (CR inflating) humanoid HD and 1-2 levels of fighter. A standard force package is 9-13 humanoids which will be about two for each PC. It's assumed that they are generally capable of recognizing spell casters by their distinctive dress and that they are trained to deal with them. While tactics vary between races, in general they'll make a lose formation with 4-6 'heavy infantry' in the middle equipped with shields and the best armor the force can afford, and who generally fight defensively and try to hold the PC's in place by forming a line and closing. They don't worry too much about causing damage immediately, the goal is grab attention and maximize AC. Shield wall rules if you have them. The heavy infantry are generally flanked by 2-4 skirmishers, close enough to the line to provoke AoO for anything that tries to go past and optimized for initiative, evasion, and delivering AoO. They usually have spears of several sizes so to both attack at range and threaten an area. They are trying to stop flankers and/or flank the PC's themselves. Levels in rogue can be effective here. Behind the skirmishers on the ends of the main line and spread out are 2-4 archers, optimized for rate of fire and accuracy and with a light backup melee weapon. There initial job is to ready actions to interrupt spell casters. Anyone that tries to cast anything gets arrows going their way. If anyone gets into the back field, there job switches to teaming with the skirmishers and/or any unengaged heavy infantry to beat down the isolated PC. Finally, sandwiched between the archers and behind the front line are 1-3 low level spellcasters. They have spell lists divided along the following lines - battlefield control (area of effect save or suck things, if you want to be abusive, web or entanglement for example), group buffs, and dispels/counters. So essentially we are talking about usually 9-13 non-elite, reasonably equipped but not up the level of elite NPCs, 1st-3rd level characters. I generally rate them at about 1 less CR than their level (because they aren't elite and don't have the equipment expected of an elite NPC). If you really need to crank it up, you can add some 4th to 5th level leaders or elite forces, particularly among the spell casters. Well played, this can challenge PC's anywhere up to high level play, and the same basic theory could be applied to things like ogres, trolls, or giants to keep you going to anywhere up to epic level. Or you can do mixed groups, like say ogre zombies, ghasts, cultist archers, two evil clerics and a necromancer.
And that's just one mode. Another tactical mode is the 'all archer team', about the same number of foes, but instead they spread out with about 20' between archers and try to begin the engagement beyond normal medium range of a spellcaster (beyond 200 feet). Preferably they take partial cover - tree boles, prepared fighting positions, etc. Part of the team readies actions against spellcasters, the rest tries to harrass the team (concentrate fire on single PC's for maximum cheese). In response to attempts to engage individual members of the team, each archer instead tries to fall back using full evasion actions or just running if needed. Don't pull this too often, but the first time you do, it _will_ be remembered.
You can also do an all 'Skirmisher Team', though the best way to pull this is with cavalry - goblins on worgs are my favorite, but hobgoblins on horses are good too. The goal here is to try to prevent the PC's from successfully engaging, negate there ability to use full attack actions, negate thier advantage in the action economy, and use the mounts as both weapons and hit point buffers. Teach the PC's the value of mobility, make them hate ride by attack, and use their own beat down strategy of concentrating attacks on a single foe against them. Oh, and while I'm on it, any intelligent foe that realize he's the single target of all attacks and who has allies is going to make defensive attacks or take the full defensive action. Just don't tell the Pc's that. Make them figure it out.
If you have a narrow front, like a corridor, you can also do all heavy infantry units fighting in ranks with pole arms and the front line going defensive, though that tends to play to the PC's strengths if you just have the options in RAW.
If you don't mind a really long combat, combine one or more tactical teams in the same combat - either the same or varied groups. Those of you that have played 4e probably have realized that they tried to design the game to teach you to do this very thing, but its not a distinctive feature of 4e. You can find prototypical humanoid force packages all the way back to Gygax.
Next post, I'll talk about the tactical considerations of summoned monsters.
